mybatis 动态拼接sql

MyBatis提供了动态SQL的支持，可以根据不同的条件生成不同的SQL语句，具体实现方式有以下几种：

1. if标签

<select id="getUser" resultType="User">
    select * from user where 1=1
    <if test="userName != null">
        and user_name = #{userName}
    </if>
    <if test="gender != null">
        and gender = #{gender}
    </if>
</select>

2. choose/when/otherwise标签

<select id="getUser" resultType="User">
    select * from user where 1=1
    <choose>
        <when test="userName != null">
            and user_name = #{userName}
        </when>
        <when test="gender != null">
            and gender = #{gender}
        </when>
        <otherwise>
            and age > 18
        </otherwise>
    </choose>
</select>

3. where标签

<select id="getUser" resultType="User">
    select * from user
    <where>
        <if test="userName != null">
            and user_name = #{userName}
        </if>
        <if test="gender != null">
            and gender = #{gender}
        </if>
    </where>
</select>

4. set标签（用于更新操作）

<update id="updateUser" parameterType="User">
    update user
    <set>
        <if test="userName != null">
            user_name = #{userName},
        </if>
        <if test="gender != null">
            gender = #{gender},
        </if>
    </set>
    where id = #{id}
</update>

以上是MyBatis动态SQL的常见实现方式，可以根据实际情况选择使用。