Preconditions

Defines if the database executed against matches the type specified.

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

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

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

• property – the name of the property to check. (required)
• value – the required value for a given property.

Defines if the specified changeset has already been executed.

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

• Id – the changeset id. (required)
• author – the changeset author. (required)
• changelog-file – the file name (including searchPath relative path) of the changeset. (required)

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.

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

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

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

• expectedRows – the number of rows the user expects in a table (required)
• tableName – the name of the column's table (required)

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

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

Defines if the specified column exists in the database.

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

• schemaName – the name of the table's schema.
• tableName – the name of the column's table. (required)
• columnName – the name of the column. (required)

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

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

• schemaName – the name of the foreign key's schema.
• foreignKeyName – the name of the foreign key. (required)
• foreignKeyTableName – the name of the table the foreign key is in. (required in 4.7.0+)

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

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

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

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

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

• schemaName – the name of the index's schema.
• indexName – the name of the index.
• tableName – the name of the table.
• columnNames – the name of the column.

Defines if the specified primary key exists in the database.

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

• schemaName – the name of the primary key's schema.
• primaryKeyName – the name of the primary key constraint.
• tableName – the name of the table containing primary key.
  (tableName or primaryKeyName is required)

Defines if the specified sequence exists in the database.

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

• schemaName – the name of the sequences' schema.
• sequenceName – the name of the sequence. (required)

Defines if the specified table exists in the database.

<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>

• schemaName – the name of the table's schema.
• tableName – the name of the table. (required)

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

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

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

• constraintName – the name of the unique constraint
• tableName – the name of the column's table (required)
• columnNames – the name of the column

Defines if the specified view exists in the database.

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

• schemaName – the name of the view's schema.
• viewName – the name of the view. (required)

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.

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

className – the name of the custom precondition class. (required)

The customPrecondition sub-tags:

• param – the parameter to pass to the custom precondition.
• name – the name of the parameter to set. (required)
• value – a string value to set parameter to. (required)