Detailed explanation of MyBatis dynamic SQL tags: conditional judgment tags

As an excellent persistence layer framework, MyBatis provides rich and flexible dynamic SQL syntax, which can be used according to different Generate different SQL statements based on the conditions to meet various complex query needs. Among them, the conditional judgment tag is one of them, which can generate SQL statements based on the true or false of the condition. This article will introduce in detail the usage of conditional judgment tags in MyBatis and provide specific code examples.

1. Tag

The tag is one of the tags used for conditional judgment in MyBatis. It can control the generation of SQL statements based on the true or false of the condition. The specific syntax is as follows:

<select id="selectUsers" parameterType="map" resultType="User">
    SELECT * FROM user
    <where>
        <if test="name != null">
            AND name = #{name}
        </if>
        <if test="age != null">
            AND age = #{age}
        </if>
    </where>
</select>

In the above example, the tag is used to determine whether the incoming parameters contain the name and age fields. If they exist, the corresponding conditions will be added to the SQL statement. . Among them, the test attribute is used to specify the expression for conditional judgment. The content inside the tag will be executed only when the expression is true.

2. tag

The tag is a tag used for multiple conditional judgments, similar to the switch statement in Java. It contains multiple and an tag, and only the content inside the first tag that is true will be executed. Specific examples are as follows:

<select id="selectUsers" parameterType="map" resultType="User">
    SELECT * FROM user
    <where>
        <choose>
            <when test="name != null">
                AND name = #{name}
            </when>
            <when test="age != null">
                AND age = #{age}
            </when>
            <otherwise>
                AND id = #{id}
            </otherwise>
        </choose>
    </where>
</select>

In the above example, the tag is used to determine whether the incoming parameters contain the name and age fields. If so, different SQL statements will be generated based on the conditions. If neither is satisfied, the content inside the tag will be executed.

3. tag

tag is a tag used to generate WHERE clauses. It can automatically remove redundant AND and OR keywords and determine whether Add WHERE keyword. The specific usage is as follows:

<select id="selectUsers" parameterType="map" resultType="User">
    SELECT * FROM user
    <where>
        <if test="name != null">
            AND name = #{name}
        </if>
        <if test="age != null">
            AND age = #{age}
        </if>
    </where>
</select>

In the above example, the tag will automatically remove the AND keyword between the name and age conditions, and determine whether to add the WHERE keyword based on the conditions, which can simplify the SQL statement of preparation.

4. Tag

The tag is also a tag used to generate SQL statements. It can remove redundant commas or connection characters such as AND and OR. Specific examples are as follows:

<select id="selectUsers" parameterType="map" resultType="User">
    SELECT * FROM user
    <where>
        <trim prefix="WHERE" suffixOverrides="AND">
            <if test="name != null">
                AND name = #{name}
            </if>
            <if test="age != null">
                AND age = #{age}
            </if>
        </trim>
    </where>
</select>

In the above example, the tag will remove redundant AND keywords and add the WHERE keyword when the conditions are met, which can effectively handle the splicing problem of SQL statements.

Through the above introduction, we have a detailed understanding of the usage of conditional judgment tags and specific code examples in MyBatis. These tags can help us efficiently generate complex SQL statements and improve development efficiency. I hope this article is helpful to you, thank you for reading!

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