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动态sql，顾名思义，就是动态的在xml中拼接sql语句。

1、where

<select id="getUserById" resultType="com.guor.bean.User">
  SELECT * FROM user 
  <where>
    <if test="position != null">
      AND position like #{position}
    </if>
    <if test="school != null">
      AND school like #{school}
    </if>
  </where>
</select>

注：where元素只有至少一个条件存在时，才会有where，而且，如果开头是and或者or时，where元素会自动将其舍弃。

2、trim

其实where也可以用trim来改写：

<select id="getUserById" resultType="com.guor.bean.User">
  SELECT * FROM user 
  <trim prefix="WHERE" prefixOverrides="AND |OR ">
    <if test="position != null">
      AND position like #{position}
    </if>
    <if test="school != null">
      AND school like #{school}
    </if>
  </trim>
</select>

3、set

set元素会动态前置set关键字，同时也会删除无关的逗号。

<update id="updateUserById">
  update user
    <set>
      <if test="username != null">username=#{username},</if>
      <if test="password != null">password=#{password},</if>
      <if test="email != null">email=#{email},</if>
      <if test="address != null">ddress=#{ddress}</if>
    </set>
  where id=#{id}
</update>

set也可以用trim进行改写：

<update id="updateUserById">
  update user
    <trim prefix="SET" suffixOverrides=",">
      <if test="username != null">username=#{username},</if>
      <if test="password != null">password=#{password},</if>
      <if test="email != null">email=#{email},</if>
      <if test="address != null">ddress=#{ddress}</if>
    </trim>
  where id=#{id}
</update>

4、if

<select id="getUserById" resultType="com.guor.bean.User">
  SELECT * FROM user WHERE 1 = 1 
  <if test="position != null">
    AND position like #{position}
  </if>
</select>

5、choose、when、otherwish

<select id="getUsersById" resultType="com.guor.bean.User">
  SELECT * FROM user WHERE 1 = 1 
  <choose>
    <when test="position != null">
      AND position like #{position}
    </when>
    <when test="department != null and department.name != null">
      AND department_name like #{department.name}
    </when>
    <otherwise>
      AND department_name = '公司'
    </otherwise>
  </choose>
</select>

6、foreach

<select id="selectUserById" resultType="com.guor.bean.User">
  SELECT id,name,age,sex,address,school FROM user u
  WHERE id in
  <foreach item="item" index="index" collection="list"
      open="(" separator="," close=")">
        #{item}
  </foreach>
</select>

当使用可迭代对象或者数组时，index 是当前迭代的次数，item 的值是本次迭代获取的元素。

当使用 Map 对象（或者 Map.Entry 对象的集合）时，index 是键，item 是值。

7、bind

<select id="selectUserByDept" resultType="com.guor.bean.User">
  <bind name="dept" value="'%' + _department.getId() + '%'" />
  SELECT * FROM user
  WHERE dept_id = #{dept}
</select>

8、连接多数据库

<insert id="getCurrentTime">
    <if test="_databaseId == 'oracle'">
      select to_char(sysdate,'yyyy-mm-dd hh24:mi:ss') from dual
    </if>
    <if test="_databaseId == 'mysql'">
      select now() as Systemtime
    </if>
</insert>

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原文链接:https://blog.csdn.net/guorui_java/article/details/117792065