Detailed explanation of how to use SQL fragment multiplexing in MyBatis

1. Advantages of SQL fragment multiplexing

The maintainability of code is an important consideration in software development. When the same SQL fragment is scattered across multiple places, if modifications are needed, such as changing the field name or adding new public fields, the involved SQL statements must be modified one by one, which undoubtedly increases the risk of errors and is inefficient. By reusing SQL fragments, you only need to modify them at the one where the fragment is defined, and all SQL statements referring to the fragment will be changed accordingly, greatly improving the maintainability of the code. At the same time, it also makes the SQL code structure clearer and improves readability.

2. Define SQL fragments

In the XML mapping file of MyBatis, use the <sql> tag to define SQL snippets. For example, suppose we have a User table that contains id, username, password and email fields. If we often need to query fields such as id, username and email, we can define SQL fragments as follows:

<sql >
    id, username, email
</sql>

The id attribute userFields here is the unique identifier of this SQL fragment, and will operate according to this identifier when subsequent references.

3. Quoting SQL fragments

In SELECT statements or other places where this fragment is required, use the <include> tag to reference. For example, to query the id, username and email information of all users, SQL statements can be written like this:

<select  resultMap="userResultMap">
    SELECT <include ref/>
    FROM users
</select>

in<include ref/>Will define the previously defineduserFieldsThe content of SQL fragment (i.e.id, username, email) Insert to the currentSELECTIn the statement.

4. Use SQL fragments in dynamic SQL

SQL fragments can also play a role in dynamic SQL scenarios. For example, there is a way to query users based on conditions, sometimes just queryidandusername, sometimes you need to queryid、usernameandemail. Dynamic SQL can be written as follows:

<select  resultMap="userResultMap">
    SELECT
    <choose>
        <when test="includeEmail">
            <include ref/>
        </when>
        <otherwise>
            id, username
        </otherwise>
    </choose>
    FROM users
    WHERE 1 = 1
    <if test="username!= null">
        AND username LIKE '%${username}%'
    </if>
</select>

In this example, the value of the includeEmail parameter determines whether to refer to the SQL fragment containing the email field. If includeEmail is true, the userFields fragment is referenced and the id, username and email fields are queryed; otherwise, only the id and username fields are queryed, and the dynamic SQL part is also included in the filtering according to the username condition.

The above is the detailed explanation of the usage method of SQL fragment reuse in MyBatis. For more information about MyBatis SQL fragment reuse, please pay attention to my other related articles!