What is the purpose of the apply function in mybatis?

In MyBatis, the element is used to dynamically generate a group of parameters within an SQL statement. Typically, this element is used in conjunction with the element to iterate through elements in a collection or array, and apply the iteration results to the SQL statement.

For example, we can iterate through the elements of a List collection and use the elements to apply the iteration result to the IN clause of an SQL statement.

The sample code is as follows:

<select id="selectUsersByIds" resultType="User" parameterType="Map">
  SELECT * FROM users
  WHERE id IN
  <apply>
    <foreach collection="ids" item="id" separator="," open="(" close=")">
      #{id}
    </foreach>
  </apply>
</select>

In the example above, the element is used to iterate through the elements in the ids collection in the Map, and then the element applies the iteration result to the IN clause in the SQL statement. This allows for dynamically generating an IN clause containing all elements in the ids collection, achieving the functionality of dynamically constructing SQL statements.