Preconditions

Preconditions are tags you add to your changelog or individual changesets to control the execution of an update based on the state of the database. Preconditions let you specify security and standardization requirements for your changesets. If a precondition on a changeset fails, Liquibase does not deploy that changeset.

Uses

You can use preconditions to:

Document what assumptions the author of the changelog had when creating it.
Enforce that those assumptions are not violated by users running the changelog.
Perform data checks before performing an unrecoverable change such as dropTable.
Control what changesets are run and not run based on the state of the database.

You can use all Liquibase preconditions in XML, YAML, and JSON changelogs. Only some preconditions are supported in SQL changelogs. For a list of preconditions, see Available preconditions.

Syntax

You can use one local preConditions tag per changeset and one global preConditions tag in the changelog (outside any changeset). In XML, YAML, and JSON changelogs, you can use conditional logic to add multiple preconditions within a single preConditions tag.

Preconditions at the changelog level are applied to all changesets, not just those listed in the current changelog or its child changelogs.

The following example XML, YAML, and JSON changelogs will only be run if the database executed against is Oracle and the database user executing the script is SYSTEM. Also, they will run the changeset with the sqlCheck precondition and createTable. The example SQL changelog only runs the sqlCheck precondition and createTable.

If the preconditions check fails, you will receive a warning and it will continue executing the changeset as normal because of the onFail="WARN" precondition. To prevent the execution of the changeset when the precondition fails, you can set HALT or CONTINUE values. For more information, see onFail/onError precondition values.

Example changelogs with preconditions

--liquibase formatted sql
			
--changeset Liquibase User:1
--preconditions onFail:WARN 
--precondition-sql-check expectedResult:0 SELECT COUNT(*) FROM example_table
--comment: /* Comments should go after the precondition. Otherwise, Liquibase returns an error. */
create table example_table (  
	id int primary key,
	firstname varchar(50),
	lastname varchar(50) not null,
	state char(2)
)

databaseChangeLog:
 - preConditions:
     - dbms:
        type: h2
     - runningAs:
        username: dbuser
 - changeSet:
     id:  1
     author: Liquibase User
     preConditions:
       - onFail: WARN
       - not:
          - tableExists:
              tableName: example_table
     comment: Comments should go after the precondition. Otherwise, Liquibase returns an error.
     changes:
     - createTable:
         tableName: example_table
         columns:
          - column:
             name: id
             type: int
             autoIncrement: true
             constraints:
              primaryKey: true
              nullable: false
          - column:
             name:  firstname
             type:  varchar(50)
          - column:
             name:  lastname
             type:  varchar(50)
             constraints:
              nullable:  false
          - column:
             name:  state
             type:  char(2)
 - changeSet:
     id:  2
     author: Liquibase User
     preConditions:
       - onFail: WARN
       - sqlCheck:
          expectedResult: 0
          sql: SELECT COUNT(*) FROM example_table
     comment: Comments should go after the precondition. Otherwise, Liquibase returns an error.
     changes:
     - insert:
        tableName:  example_table
        columns:
        -  column:
            name:  firstname
            value:  John
        -  column:
            name:  lastname
            value:  Doe
        -  column:
            name:  state
            value:  TX

{
  "databaseChangeLog": [
    {
      "preConditions": [
        {
          "dbms": {
            "type": "h2"
          }
        },
        {
          "runningAs": {
            "username": "dbuser"
          }
        }
      ],
      "changeSet": [
        {
          "id": "1",
          "author": "Liquibase User",
          "preConditions": [
            {
              "onFail": "WARN"
            },
            {
              "not": [
                {
                  "tableExists": {
                    "tableName": "example_table"
                  }
                }
              ]
            }
          ],
          "comment": "Comments should go after the precondition. Otherwise, Liquibase returns an error.",
          "changes": [
            {
              "createTable": {
                "tableName": "example_table",
                "columns": [
                  {
                    "column": {
                      "name": "id",
                      "type": "int",
                      "autoIncrement": true,
                      "constraints": {
                        "primaryKey": true,
                        "nullable": false
                      }
                    }
                  },
                  {
                    "column": {
                      "name": "firstname",
                      "type": "varchar(50)"
                    }
                  },
                  {
                    "column": {
                      "name": "lastname",
                      "type": "varchar(50)",
                      "constraints": {
                        "nullable": false
                      }
                    }
                  },
                  {
                    "column": {
                      "name": "state",
                      "type": "char(2)"
                    }
                  }
                ]
              }
            }
          ]
        }
      ]
    }
  ]
}

<?xml version="1.1" encoding="UTF-8" standalone="no"?>
<MadCap:snippetText src="../../Z_Resources/Snippets/code/liquibase-xsd-xml-changelog-closed.flsnp" />

    <preConditions>
        <dbms  type="h2"  />
        <runningAs  username="dbuser"  />
    </preConditions>

    <changeSet  id="1"  author="Liquibase User">
        <preConditions  onFail="WARN">
            <not>
                <tableExists tableName="example_table" />
            </not>
        </preConditions>
        <comment>Comments should go after the precondition. Otherwise, Liquibase returns an error.</comment>

        <createTable  tableName="example_table">
            <column name="id" type="int" autoIncrement="true">
                <constraints primaryKey="true" nullable="false"/>
            </column>
            <column name="firstname" type="varchar(50)"/>
            <column name="lastname" type="varchar(50)">
                <constraints nullable="false"/>
            </column>
            <column name="state" type="char(2)"/>
        </createTable>
    </changeSet>

    <changeSet id="2" author="Liquibase User">
        <preConditions>
            <sqlCheck expectedResult="0">SELECT COUNT(*) FROM example_table</sqlCheck>
        </preConditions>
        <insert tableName="example_table">
            <column name="firstname" value="John"/>
            <column name="lastname" value="Doe"/>
            <column name="state" value="TX"/>
        </insert>
    </changeSet>

</databaseChangeLog>

Multiple preconditions

In XML, JSON, and YAML changelogs, you can set multiple preconditions in one preConditions tag by using nestable AND, OR, and NOT conditional logic tags. If no conditional tags are specified, the default value is AND.

Liquibase uses lazy evaluation (call-by-need) when running preconditions. For example, if the first condition of an AND expression fails, Liquibase does not evaluate the second condition. If the first condition of an OR expression succeeds, Liquibase does not evaluate the second condition.

Syntax for multiple preconditions:

- preConditions:
    - or:
        - CONDITION_1: VALUE
        - CONDITION_2:
           SUB_CONDITION: SUB_VALUE

For example, the following syntax example checks that the update is running on Oracle and with the SYSTEM user but will only generate a warning message if the precondition fails:

- preConditions:
    - onFail: WARN
    - dbms:
       type: oracle
    - runningAs:
       username: SYSTEM

The following requires the changelog to run on Oracle or MySQL:

- preConditions:
    - or:
       - dbms:
          type: oracle
       - dbms:
          type: mysql

You can also see the precondition running as SYSTEM if executing against an Oracle database or running as SA if executing against a Microsoft SQL Server database:

- preConditions:
    - or:
       - and:
          - dbms:
             type: oracle  
             runningAs:
              username: SYSTEM
       - and:
          - dbms:
             type: mssql
             runningAs:
              username: sa

Syntax for multiple preconditions:

{
  "preConditions": [
    {
      "or": [
        {
          "CONDITION_1": "VALUE"
        },
        {
          "CONDITION_2": {
            "SUB_CONDITION": "SUB_VALUE"
          }
        }
      ]
    }
  ]
}

For example, the following syntax example checks that the update is running on Oracle and with the SYSTEM user but will only generate a warning message if the precondition fails:

{
  "preConditions": [
    {
      "dmbs": {
        "type": "oracle"
      }
    },
    {
      "runningAs": {
        "username": "SYSTEM"
      }
    }
  ]
}

The following requires the changelog to run on Oracle or MySQL:

{
  "preConditions": [
    {
      "or": [
        {
          "dbms": {
            "type": "oracle"
          }
        },
        {
          "dbms": {
            "type": "mysql"
          }
        }
      ]
    }
  ]
}

You can also see the precondition running as SYSTEM if executing against an Oracle database or running as SA if executing against a Microsoft SQL Server database:

{
  "preConditions": [
    {
      "or": [
        {
          "and": [
            {
              "dbms": {
                "type": "oracle"
              }
            },
            {
              "runningAs": {
                "username": "SYSTEM"
              }
            }
          ]
        },
        {
          "and": [
            {
              "dbms": {
                "type": "mssql"
              }
            },
            {
              "runningAs": {
                "username": "sa"
              }
            }
          ]
        }
      ]
    }
  ]
}

Syntax for multiple preconditions:

<preConditions>
    <or>
        <CONDITION_1  SUB_CONDITION_1="SUB_VALUE_1">
        <CONDITION_2  SUB_CONDITION_2="SUB_VALUE_2">
    </or>
</preConditions>

For example, the following syntax example checks that the update is running on Oracle and with the SYSTEM user but will only generate a warning message if the precondition fails:

<preConditions  onFail="WARN">
    <dbms  type="oracle"  />
    <runningAs  username="SYSTEM"  />
</preConditions>

The following requires the changelog to run on Oracle or MySQL:

<preConditions>
   <or>
      <dbms  type="oracle"  />
      <dbms  type="mysql"  />
   </or>
</preConditions>

You can also see the precondition running as SYSTEM if executing against an Oracle database or running as SA if executing against a Microsoft SQL Server database:

<preConditions>
    <or>
       <and>
          <dbms  type="oracle"  />  
          <runningAs  username="SYSTEM"  />
       </and>
       <and>
          <dbms  type="mssql"  />
          <runningAs  username="sa"  />
       </and>
     </or>
</preConditions>

Handling failures and errors

Liquibase defines two types of preconditions:

Precondition failures which represent that the check failed
Precondition errors that are the exceptions thrown in the execution of a check

The process of both can be controlled through the onFail and onError attributes on the preConditions tag.

Available attributes

Attribute	Description
`onError`	Controls what happens if there is an error checking whether the precondition passed or not.
`onErrorMessage`	Provides a custom message to output when preconditions fail. Since 2.0
`onFail`	Controls what happens if the preconditions check fails.
`onFailMessage`	Provides a custom message to output when preconditions fail. Since 2.0
`onSqlOutput`	Controls how preconditions are evaluated with the `update-sql` command for XML, YAML, and JSON changelogs. Since 1.9.5
`onUpdateSql`	Controls how preconditions are evaluated with the `update-sql` command for formatted SQL changelogs.

`onFail`/`onError` precondition values

Value	Description
`CONTINUE`	Skips over the changeset. Execution of the changeset will be attempted again on the next update. Continues with the changelog.
`HALT`	Halts the execution of the entire changelog (default). `HALT` can be put outside a changeset (e.g. at the beginning of the changelog).
`MARK_RAN`	Skips over the changeset but mark it as executed. Continues with the changelog.
`WARN`	Sends a warning and continues executing the changeset / changelog as normal. `WARN` can be put outside a changeset (e.g. at the beginning of the changelog).

`onSqlOutput`/`onUpdateSql` values

Value	Description
`FAIL`	When you run the `update-sql` command, Liquibase fails any preconditions.
`IGNORE`	When you run the `update-sql` command, Liquibase ignores any preconditions.
`TEST`	When you run the `update-sql` command, Liquibase evaluates preconditions normally.

Failures and errors will cause Liquibase to return non-zero exit codes in command output. Warnings will cause Liquibase to return exit codes of zero.

Example failures/errors

The following is a sample failure on a changelog:

Unexpected error running Liquibase: Validation Failed:
1 preconditions failed
myChangelog.xml : DBMS Precondition failed: expected oracle, got postgresql

The following is a sample error on a changelog:

Unexpected error running Liquibase: Validation Failed:
1 preconditions generated an error
myChangelog.xml : liquibase.precondition.core.SqlPrecondition@5745ca0e : ERROR: relation "public.table_does_not_exist" does not exist

The following is a sample warning on a changeset:

WARNING: Executing myChangelog.xml::01-new::wonder.woman despite precondition failure due to onFail='WARN':
1 preconditions failed
myChangelog.xml : Column public.customer.name_new does not exist

Available preconditions

The examples of preconditions shown in the table doesn't include the schemaName attribute. It is best practice not to keep the schemaName in the changeset and rely on the default schema except for times you don't want to create something in the default schema.

Additionally, you can include the onError or onFail attributes with the WARN, HALT, CONTINUE, or MARK_RAN value in the preconditions tag, however, the CONTINUE and MARK_RAN options can only be applied to preconditions inside a changeset.

`changeLogPropertyDefined`

Checks whether given changelogattribute is present. It fails if the value is not the same as given.

Attribute	Description	Requirement
`property`	The name of the property to check.	Required
`value`	The required value for a given property.	Optional

- preConditions:
    - onFail: WARN
    - changeLogPropertyDefined:
       property: myproperty
       value: requiredvalue

{
  "preConditions": [
    {
      "onFail": "WARN"
    },
    {
      "changeLogPropertyDefined": {
        "property": "myproperty",
        "value": "requiredvalue"
      }
    }
  ]
}

<preConditions onFail="WARN">
    <changeLogPropertyDefined property="myproperty"
                              value="requiredvalue"/>
</preConditions>

`changeSetExecuted`

Defines if the specified changeset has already been executed.

Attribute	Description	Requirement
`id`	The changeset id.	Required
`author`	The changeset author.	Required
`changeLogFile`	The file name (including searchPath relative path) of the changeset.	Required

- preConditions:
    - onFail: HALT
    - changeSetExecuted:
       id: 1
       author: liquibase
       changeLogFile: changelog.xml

{
  "preConditions": [
    {
      "onFail": "HALT"
    },
    {
      "changeSetExecuted": {
        "id": "1",
        "author": "liquibase",
        "changeLogFile": "changelog.xml"
      }
    }
  ]
}

<preConditions onFail="HALT">
    <changeSetExecuted id="1"
                          author="liquibase"
                          changeLogFile="changelog.xml" />
</preConditions>

`columnExists`

Defines if the specified column exists in the database.

Attribute	Description	Requirement
`columnName`	The name of the column.	Required
`tableName`	The name of the column's table.	Required
`schemaName`	The name of the table's schema.	Optional

- preConditions:
    - onFail: WARN
    - columnExists:
       tableName: table1
       columnName: column1

{
  "preConditions": [
    {
      "onFail": "WARN"
    },
    {
      "columnExists": {
        "tableName": "table1",
        "columnName": "column1"
      }
    }
  ]
}

<preConditions onFail="WARN">
    <columnExists tableName="table1"
                  columnName="column1" />
</preConditions>

`dbms`

Defines if the database executed against matches the type specified.

Attribute	Description	Requirement
`type`	The type of database expected. Multiple dbms values can be specified using comma-separated values.	Required

- preConditions:
    - onFail: WARN
    - or:
       - dbms:
          type: oracle
       - dbms:
          type: mysql

{
  "preConditions": [
    {
      "onFail": "WARN"
    },
    {
      "or": [
        {
          "dbms": {
            "type": "oracle"
          }
        },
        {
          "dbms": {
            "type": "mysql"
          }
        }
      ]
    }
  ]
}

<preConditions onFail="WARN">
  <or>
    <dbms  type="oracle"  />
    <dbms  type="mysql"  />
  </or>
</preConditions>

`expectedQuotingStrategy`

Controls how object names are quoted in the SQL files generated by Liquibase and used in calls to the database. For more information, see objectQuotingStrategy.

Attribute	Description	Requirement
`strategy`	The object quoting strategy. Valid values are `LEGACY`, `QUOTE_ALL_OBJECTS`, and `QUOTE_ONLY_RESERVED_WORDS`.	Required

- preConditions:
    - onFail: WARN
    - expectedQuotingStrategy:
       strategy: LEGACY

{
  "preConditions": [
    {
      "onFail": "WARN"
    },
    {
      "expectedQuotingStrategy": {
        "strategy": "LEGACY"
      }
    }
  ]
}

<preConditions onFail="WARN">
    <expectedQuotingStrategy strategy="LEGACY" />
</preConditions>

`foreignKeyConstraintExists`

Defines if the specified foreign key exists in a table in the database.

Attribute	Description	Requirement
`foreignKeyName`	The name of the foreign key.	Required
`foreignKeyTableName`	The name of the table the foreign key is in.	Required in 4.7.0+
`schemaName`	The name of the foreign key's schema.	Optional

- preConditions:
    - onFail: WARN
    - foreignKeyConstraintExists:
       foreignKeyName: PK_2354
       foreignKeyTableName: myTable

{
  "preConditions": [
    {
      "onFail": "WARN"
    },
    {
      "foreignKeyConstraintExists": {
        "foreignKeyName": "PK_2354",
        "foreignKeyTableName": "myTable"
      }
    }
  ]
}

<preConditions onFail="WARN">
    <foreignKeyConstraintExists foreignKeyName="PK_2354"
                                foreignKeyTableName="myTable" />
</preConditions>

`indexExists`

Defines if the specified index exists in the database. You can either specify the indexName attribute or tableName and columnNames attributes.

Note: There are a few databases where the indexName is not unique, which is why both indexName and tableName can be used.

Attribute	Description	Requirement
`indexName`	The name of the index.	Optional
`columnNames`	The name of the column.	Optional
`tableName`	The name of the table.	Optional
`schemaName`	The name of the index's schema.	Optional

- preConditions:
    - onFail: WARN
    - indexExists:
       indexName: originindex

- preConditions:
    - onFail: WARN
    - indexExists:
       tableName: MY_NEW_TABLE
       columnNames: ID

- preConditions:
    - onFail: WARN
    - indexExists:
       indexName: originindex
       tableName: MY_NEW_TABLE

{
  "preConditions": [
    {
      "onFail": "WARN"
    },
    {
      "indexExists": {
        "indexName": "originindex"
      }
    }
  ]
}

{
  "preConditions": [
    {
      "onFail": "WARN"
    },
    {
      "indexExists": {
        "tableName": "MY_NEW_TABLE",
        "columnNames": "ID"
      }
    }
  ]
}

{
  "preConditions": [
    {
      "onFail": "WARN"
    },
    {
      "indexExists": {
        "indexName": "originindex",
        "tableName": "MY_NEW_TABLE"
      }
    }
  ]
}

<preConditions onFail="WARN">
    <indexExists indexName="originindex"/>
</preConditions>

<preConditions onFail="WARN">
    <indexExists tableName="MY_NEW_TABLE"
                 columnNames="ID"/>
</preConditions>

<preConditions onFail="WARN">
    <indexExists indexName="originindex"
                 tableName="MY_NEW_TABLE"/>
</preConditions>

`primaryKeyExists`

Defines if the specified primary key exists in the database.

Attribute	Description	Requirement
`primaryKeyName`	The name of the primary key constraint.	Either `tableName` or `primaryKeyName` is required
`tableName`	The name of the table containing primary key.	Either `tableName` or `primaryKeyName` is required Note: On DB2, H2, HSQLDB, MySQL, and SQLite, `tableName` is required even if you also specify `primaryKeyName`.
`schemaName`	The name of the primary key's schema.	Optional

- preConditions:
    - onFail: HALT
    - primaryKeyExists:
       primaryKeyName: ID_2354
       tableName: table1

{
  "preConditions": [
    {
      "onFail": "HALT"
    },
    {
      "primaryKeyExists": {
        "primaryKeyName": "ID_2354",
        "tableName": "table1"
      }
    }
  ]
}

<preConditions onFail="HALT">
    <primaryKeyExists primaryKeyName="ID_2354"
                      tableName="table1" />
</preConditions>

`rowCount`

Checks that the number of rows in a table matches an expected value.

Attribute	Description	Requirement
`expectedRows`	The number of rows the user expects in a table.	Required
`tableName`	The name of the column's table.	Required

- preConditions:
    - onFail: WARN
    - rowCount:
       expectedRows: 5
       tableName: person

{
  "preConditions": [
    {
      "onFail": "WARN"
    },
    {
      "rowCount": {
        "expectedRows": "5",
        "tableName": "person"
      }
    }
  ]
}

<preConditions onFail="WARN">
    <rowCount expectedRows="5"
              tableName="person"/>
</preConditions>

`runningAs`

Defines if the database user executed under matches the username specified.

Attribute	Description	Requirement
`username`	The database user script which is expected to run as.	Required

- preConditions:
    - onFail: WARN
    - dbms:
       type: oracle
    - runningAs:
       username: SYSTEM

{
  "preConditions": [
    {
      "onFail": "WARN"
    },
    {
      "dbms": {
        "type": "oracle"
      }
    },
    {
      "runningAs": {
        "username": "SYSTEM"
      }
    }
  ]
}

<preConditions  onFail="WARN">
    <dbms  type="oracle"  />
    <runningAs  username="SYSTEM"  />
</preConditions>

`sequenceExists`

Defines if the specified sequence exists in the database.

Attribute	Description	Requirement
`sequenceName`	The name of the sequence.	Required
`schemaName`	The name of the sequence's schema.	Optional

- preConditions:
    - onFail: WARN
    - sequenceExists:
       sequenceName: idnumber

{
  "preConditions": [
    {
      "onFail": "WARN"
    },
    {
      "sequenceExists": {
        "sequenceName": "idnumber"
      }
    }
  ]
}

<preConditions onFail="WARN">
    <sequenceExists sequenceName="idnumber" />
</preConditions>

`sqlCheck`

Executes an SQL string and checks the returned value. The SQL must return a single row with a single value.

To check a number of rows, use the count SQL function.
To check for ranges of values, perform the check in the SQL and return a value that can be easily compared against.

Attribute	Description	Requirement
`expectedResult`	The value to compare the SQL result to.	Required
`sql`	In JSON and YAML changelogs, the SQL statement to run. In XML changelogs, insert the SQL statement directly within the `<sqlCheck></sqlCheck>` tags. IN SQL changelogs, insert the SQL statement directly in the precondition header.	Required

--changeset your.name:1
--preconditions onFail:WARN
--precondition-sql-check expectedResult:1 SELECT COUNT(1) FROM pg_tables WHERE TABLENAME = 'myRequiredTable'
create table example_table (
	id int primary key,
	firstname varchar(50),
	lastname varchar(50) not null,
	state char(2)
)

- preConditions:
    - onFail: WARN
    - sqlCheck:
       expectedResult: 1
       sql: SELECT COUNT(1) FROM pg_tables WHERE TABLENAME = 'myRequiredTable'

{
  "preConditions": [
    {
      "onFail": "WARN"
    },
    {
      "sqlCheck": {
        "expectedResult": "1",
        "sql": "SELECT COUNT(1) FROM pg_tables WHERE TABLENAME = 'myRequiredTable'"
      }
    }
  ]
}

<preConditions onFail="WARN">
    <sqlCheck expectedResult="1">
        SELECT COUNT(1)
        FROM pg_tables
        WHERE TABLENAME = 'myRequiredTable'
    </sqlCheck>
</preConditions>

`tableExists`

Defines if the specified table exists in the database.

Attribute	Description	Requirement
`tableName`	The name of the table.	Required
`schemaName`	The name of the table's schema.	Optional

Note: tableExists is supported for SQL changelogs in Liquibase 4.27.0 and later.

--changeset your.name:1
--preconditions onFail:MARK_RAN
--precondition-table-exists table:table1 schema:12345
create table table2 (
    id int primary key
);

- changeSet:
    preConditions:
      - onFail: MARK_RAN
      - not:
          - tableExists:
             tableName: MY_NEW_TABLE
    changes:
      - createTable:
          tableName: MY_NEW_TABLE
          columns:
            - column:
               name: INTVAL
               type: ${integer.type}
               constraints:
                 nullable: false

{
  "changeSet": {
    "preConditions": [
      {
        "onFail": "MARK_RAN"
      },
      {
        "not": [
          {
            "tableExists": {
              "tableName": "MY_NEW_TABLE"
            }
          }
        ]
      }
    ],
    "changes": [
      {
        "createTable": {
          "tableName": "MY_NEW_TABLE",
          "columns": [
            {
              "column": {
                "name": "INTVAL",
                "type": "${integer.type}",
                "constraints": {
                  "nullable": "false"
                }
              }
            }
          ]
        }
      }
    ]
  }
}

<changeSet author="author" id="1">
   <preConditions onFail="MARK_RAN">
       <not>
           <tableExists tableName="MY_NEW_TABLE"/>
       </not> 
   </preConditions>

   <createTable tableName="MY_NEW_TABLE">
       <column name="IDX"
               type="${integer.type}">
           <constraints nullable="false"/> 
       </column>
       <column name="INTVAL"
               type="${integer.type}"/>
   </createTable>
</changeSet>

`tableIsEmpty`

Checks whether the specified table is empty.

Attribute	Description	Requirement
`tableName`	The name of the table.	Required
`catalogName`	The name of the database catalog.	Optional
`schemaName`	The name of the database schema.	Optional

- preConditions:
    - onFail: WARN
    - tableIsEmpty:
       tableName: myTable
       catalogName: myCatalog
       schemaName: mySchema

{
  "preConditions": [
    {
      "onFail": "WARN"
    },
    {
      "tableIsEmpty": {
        "tableName": "myTable",
        "catalogName": "myCatalog",
        "schemaName": "mySchema"
      }
    }
  ]
}

<preConditions onFail="WARN">
    <tableIsEmpty tableName="myTable"
                  catalogName="myCatalog"
                  schemaName="mySchema" />
</preConditions>

`uniqueConstraintExists`

Checks for the existence of unique constraints before running the update. (since Liquibase 4.9.0)

Attribute	Description	Requirement
`tableName`	The name of the column's table.	Required
`columnNames`	The name of the column.	Optional
`constraintName`	The name of the unique constraint.	Optional

- preConditions:
    - onFail: WARN
    - uniqueConstraintExists:
       constraintName: uq_constraint_name
       tableName: test_table

- preConditions:
    - onFail: WARN
    - uniqueConstraintExists:
       tableName: test_table
       columnNames: first_name, last_name

{
  "preConditions": [
    {
      "onFail": "WARN"
    },
    {
      "uniqueConstraintExists": {
        "constraintName": "uq_constraint_name",
        "tableName": "test_table"
      }
    }
  ]
}

{
  "preConditions": [
    {
      "onFail": "WARN"
    },
    {
      "uniqueConstraintExists": {
        "tableName": "test_table",
        "columnNames": "first_name, last_name"
      }
    }
  ]
}

<preConditions onFail="WARN">
    <uniqueConstraintExists constraintName="uq_constraint_name"
                            tableName="test_table"/>
</preConditions>

<preConditions onFail="WARN">
    <uniqueConstraintExists tableName="test_table"
                            columnNames="first_name, last_name"/>
</preConditions>

`viewExists`

Defines if the specified view exists in the database.

Attribute	Description	Requirement
`viewName`	The name of the view.	Required
`schemaName`	The name of the view's schema.	Optional

Note: viewExists is supported for SQL changelogs in Liquibase 4.27.0 and later.

--changeset your.name:1
--preconditions onFail:MARK_RAN
--precondition-view-exists view:view1 schema:12345
create table table1 (
    id int primary key
);

- preConditions:
    - onFail: HALT
    - viewExists:
       viewName: liquibase

{
  "preConditions": [
    {
      "onFail": "HALT"
    },
    {
      "viewExists": {
        "viewName": "liquibase"
      }
    }
  ]
}

<preConditions onFail="HALT">
    <viewExists viewName="liquibase" />
</preConditions>

`customPrecondition`

Can be created by adding a class that implements the liquibase.precondition.CustomPrecondition interface. Parameters on custom classes are set through reflection based on the <param> sub-tags. Pass parameters as strings to the custom precondition.

Attribute	Description	Requirement
`className`	The name of the custom precondition class.	Required

`<param>` sub-tag

The parameter to pass to the custom precondition.

Attribute	Description	Requirement
`name`	The name of the parameter to set.	Required
`value`	A string value to set parameter to.	Required

- customPrecondition:
    - className: com.example.CustomTableCheck
    - param:
       name: tableName
       value: our_table
    - param:
       name: count
       value: 42

{
  "customPrecondition": [
    {
      "className": "com.example.CustomTableCheck"
    },
    {
      "param": {
        "name": "tableName",
        "value": "our_table"
      }
    },
    {
      "param": {
        "name": "count",
        "value": "42"
      }
    }
  ]
}

<customPrecondition className="com.example.CustomTableCheck">
     <param name="tableName"
            value="our_table"/>
     <param name="count"
            value="42"/>
</customPrecondition>

Preconditions

Uses

Syntax

Example changelogs with preconditions

Multiple preconditions

Handling failures and errors

Available attributes

onFail/onError precondition values

onSqlOutput/onUpdateSql values

Example failures/errors

Available preconditions

changeLogPropertyDefined

changeSetExecuted

columnExists

dbms

expectedQuotingStrategy

foreignKeyConstraintExists

indexExists

primaryKeyExists

rowCount

runningAs

sequenceExists

sqlCheck

tableExists

tableIsEmpty

uniqueConstraintExists

viewExists

customPrecondition

<param> sub-tag

`onFail`/`onError` precondition values

`onSqlOutput`/`onUpdateSql` values

`changeLogPropertyDefined`

`changeSetExecuted`

`columnExists`

`dbms`

`expectedQuotingStrategy`

`foreignKeyConstraintExists`

`indexExists`

`primaryKeyExists`

`rowCount`

`runningAs`

`sequenceExists`

`sqlCheck`

`tableExists`

`tableIsEmpty`

`uniqueConstraintExists`

`viewExists`

`customPrecondition`

`<param>` sub-tag