Mybatis动态SQL，标签大全

动态SQL常用场景

• 批量删除

  delete from t_car where id in(1,2,3,4,5,6,......这里的值是动态的，根据用户选择的
  id不同，值是不同的);
  

• 多条件查询

  哪些字段会作为查询条件是不确定的，根据用户而定
  select * from 1 t_car where brand like '丰田%' and guide_price > 30 and .....;
  

1. if 标签

   List<Car> selectByMultiCondition(@Param("brand") String brand, @Param("guidePrice") Double guidePrice, @Param("carType") String carType);
   

   <select id="selectByMultiCondition" resultType="car">
   select * from t_car where
   <if test="brand != null and brand != ''">
   brand like #{brand}"%"
   if>
   <if test="guidePrice != null and guidePrice != ''">
   and guide_price >= #{guidePrice}
   if>
   <if test="carType != null and carType != ''">
   and car_type = #{carType}
   if>
   select>
   

   存在一些连接词的问题例如 and 和 or，当brand 为空时可能出现 select * from t_car where and…这种语法报错情况，所以需要结合其他 标签使用
2. where 标签

   <select id="selectByMultiConditionWithWhere" resultType="car">
   select * from t_car
   <where>
   <if test="brand != null and brand != ''">
   and brand like #{brand}"%"
   if>
   <if test="guidePrice != null and guidePrice != ''">
   and guide_price >= #{guidePrice}
   if>
   <if test="carType != null and carType != ''">
   and car_type = #{carType}
   if>
   where>
   select>
   

   解决了if 标签存在的一些问题，where标签的作用：让where子句更加动态智能。
   所有条件都为空时，where标签保证不会生成where子句。
   自动去除某些条件前面多余的and或or，注意后面的 and 和 or 是不会被去除的
3. trim 标签
   trim标签的属性：
   • prefix：在 SQL 语句的开头添加指定的前缀。
   • suffix：在 SQL 语句的结尾添加指定的后缀。
   • prefixOverrides：去掉 SQL 语句开头的指定前缀。
   • suffixOverrides：去掉 SQL 语句结尾的指定后缀。

   <select id="selectByMultiConditionWithTrim" resultType="car">
   select * from t_car
   <trim prefix="where" suffixOverrides="and|or">
   <if test="brand != null and brand != ''">
   brand like #{brand}"%" and
   if>
   <if test="guidePrice != null and guidePrice != ''">
   guide_price >= #{guidePrice} and
   if>
   <if test="carType != null and carType != ''">
   car_type = #{carType}
   if>
   trim>
   select>
   

   所有条件为空时，不会添加前缀，比where标签更加灵活，可以去除结尾的连接词
4. set标签
   主要使用在update语句当中，用来生成set关键字，同时去掉最后多余的“,”

   int updateWithSet(Car car);
   

   <update id="updateWithSet">
   update t_car
   <set>
   <if test="carNum != null and carNum != ''">car_num = #{carNum},if>
   <if test="brand != null and brand != ''">brand = #{brand},if>
   <if test="guidePrice != null and guidePrice != ''">guide_price = #{gui
   dePrice},if>
   <if test="produceTime != null and produceTime != ''">produce_time = #
   {produceTime},if>
   <if test="carType != null and carType != ''">car_type = #{carType},i
   f>
   set>
   where id = #{id}
   update>
   

5. choose when otherwise
   这三个标签是一起使用的，类似于 if else 嵌套选择

   <select id="selectWithChoose" resultType="car">
   select * from t_car
   <where>
   <choose>
   <when test="brand != null and brand != ''">
   brand like #{brand}"%"
   when>
   <when test="guidePrice != null and guidePrice != ''">
   guide_price >= #{guidePrice}
   when>
   <otherwise>
   produce_time >= #{produceTime}
   otherwise>
   choose>
   where>
   select>
   

6. foreach 标签
   循环数组或集合，动态生成sql，比如这样的SQL：
   • 用 in 实现批量删除

     int deleteBatchByForeach(@Param("ids") Long[] ids);
     

     
     <delete id="deleteBatchByForeach">
     delete from t_car where id in
     <foreach collection="ids" item="id" separator="," open= "("   close=  ")"  >
     #{id}
     foreach>
     delete>
     

   • 用 or 实现批量删除

     <delete id="deleteBatchByForeach2">
     delete from t_car where
     <foreach collection="ids" item="id" separator="or">
     id = #{id}
     foreach>
     delete>
     

   • 批量添加

     int insertBatchByForeach(@Param("cars") List<Car> cars);
     

     <insert id="insertBatchByForeach">
     insert into t_car values
     <foreach collection="cars" item="car" separator=",">
     (null,#{car.carNum},#{car.brand},#{car.guidePrice},#{car.produceTime},#
     {car.carType})
     foreach>
     insert>
     

7. sql标签与include标签
   sql标签用来声明sql片段
   include标签用来将声明的sql片段包含到某个sql语句当中
   作用：代码复用。易维护。

   <sql id="carCols">id,car_num carNum,brand,guide_price guidePrice,produce_t
   ime produceTime,car_type carTypesql>
   <select id="selectAllRetMap" resultType="map">
   select <include refid="carCols"/> from t_car
   select>
   <select id="selectAllRetListMap" resultType="map">
   select <include refid="carCols"/> carType from t_car
   select>
   <select id="selectByIdRetMap" resultType="map">
   select <include refid="carCols"/> from t_car where id = #{id}
   select>