What are the dynamic SQL tags in mybatis?

mybatis dynamic SQL tags: 1. <if> tag; 2. <choose>, <when> and <otherwise> tags; 3. <foreach> tag; 4. <trim> ;, <where> and <set> tags; 5. <bind> tag. Detailed introduction: 1. The <if> tag is used to determine whether a certain SQL statement is included based on conditions. It is similar to the if statement in Java; 2. <choose>, <when> and <otherwise> tags, etc. wait.

The operating system for this tutorial: Windows 10 system, DELL G3 computer.

MyBatis is an excellent persistence layer framework that supports customized SQL, stored procedures and advanced mapping. In MyBatis, dynamic SQL is a very powerful feature that allows developers to build flexible SQL queries based on different conditions. MyBatis provides a variety of dynamic SQL tags for dynamically generating SQL statements at runtime. The following are commonly used dynamic SQL tags in MyBatis:

1, <if> Tag: <if> The tag is used to determine whether a certain SQL statement is included based on conditions. It is similar to if statement in Java. For example:

<select id="findUsers" resultType="User">  
  SELECT * FROM user  
  WHERE 1=1  
  <if test="name != null">  
    AND name = #{name}  
  </if>  
  <if test="age != null">  
    AND age = #{age}  
  </if>  
</select>

2, <choose>, <when> and <otherwise> Tags: These tags are used to implement a switch-case-default structure similar to Java. The <choose> tag contains multiple <when> and an <otherwise> tag. When the attribute value of the <choose> tag is true, the content in the corresponding <when> tag is executed. If none of the <when> tags have a property value of true, the contents of the <otherwise> tag are executed. For example:

<select id="findUsers" resultType="User">  
  SELECT * FROM user  
  WHERE 1=1  
  <choose>  
    <when test="name != null">  
      AND name = #{name}  
    </when>  
    <when test="age != null">  
      AND age = #{age}  
    </when>  
    <otherwise>  
      AND is_active = 1  
    </otherwise>  
  </choose>  
</select>

3, <foreach> Tag: <foreach> tag is used to traverse a collection or array in a SQL statement and generate the corresponding SQL fragment. It is often used in scenarios such as IN queries or batch inserts. For example:

<select id="findUsersByIds" resultType="User">  
  SELECT * FROM user WHERE id IN   
  <foreach item="id" index="index" collection="ids" open="(" separator="," close=")">  
    #{id}  
  </foreach>  
</select>

4, <trim>, <where> and <set> tags: These tags are used to handle extra spaces and commas in SQL statements and generate UPDATE SET clause in the statement. The <trim> tag can be used to remove extra spaces and commas, the <where> tag can be used to generate a WHERE clause, and the <set> tag can be used to generate the SET clause in an UPDATE statement. For example:

<update id="updateUser" parameterType="User">  
  UPDATE user SET   
  <set>  
    <if test="name != null">name = #{name},</if>  
    <if test="age != null">age = #{age},</if>  
    <!-- 其他属性 -->  
  </set>  
  WHERE id = #{id}  
</update>

5, <bind> Tag: <bind> tag is used to define variables in XML mapping files and reference the variables in SQL statements. This can be used to build more complex dynamic SQL statements. For example:

<bind id="userWhereClause" parameterType="map">  
  AND name = #{name}  
  AND age = #{age}  
</bind>  
  
<select id="findUserByParams" resultType="User">  
  SELECT * FROM user WHERE 1=1 <include refid="userWhereClause"/>  
</select>

These are commonly used dynamic SQL tags in MyBatis. They can help developers build flexible and dynamic SQL query statements. When using these tags, you need to pay attention to avoid SQL injection attacks and ensure the security of input parameters.

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