一、if 标签

<mapper namespace="com.powernode.mybatis.mapper.CarMapper">
    <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>
</mapper>

如果第一个条件为空，剩下两个条件不为空，报错，SQL 语法有问题，where 后面出现了 and

解决方案：where 后面添加一个恒成立的条件

此时不管 if 标签是否生效都不会影响 SQL 语句正常执行

<mapper namespace="com.powernode.mybatis.mapper.CarMapper">
    <select id="selectByMultiCondition" resultType="car">
        select * from t_car where 0 = 0
        <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>
    </select>
</mapper>

二、where 标签 

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（后面多余的 and 不会被去除）

<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>

三、tirm 标签 

trim 标签的属性：

• prefix：在 trim 标签中的语句前添加内容

• suffix：在 trim 标签中的语句后添加内容

• prefixOverrides：前缀覆盖掉（去掉）

• suffixOverrides：后缀覆盖掉（去掉）

如果所有条件为空，where 不会被加上

<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>

四、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 “,”

比如只更新提交的不为空的字段，如果提交的数据是空或者""，那么这个字段我们将不更新。

<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 = #{guidePrice},
    </if>
    <if test="produceTime != null and produceTime != ''">
        produce_time = #{produceTime},
    </if>
    <if test="carType != null and carType != ''">
        car_type = #{carType},
    </if>
  </set>
  where id = #{id}
</update>

五、choose when otherwise

这三个标签是在一起使用的

<choose>
  <when></when>
  <when></when>
  <when></when>
  <otherwise></otherwise>
</choose>

等同于

if(){
    
}else if(){
    
}else if(){
    
}else if(){
    
}else{

}

只有一个分支会被选择！！！！  

六、foreach 标签

循环数组或集合，动态生成 sql，比如这样的 SQL：

delete from t_car where id in(1,2,3);
delete from t_car where id = 1 or id = 2 or id = 3;

insert into t_car values
  (null,'1001','凯美瑞',35.0,'2010-10-11','燃油车'),
  (null,'1002','比亚迪唐',31.0,'2020-11-11','新能源'),
  (null,'1003','比亚迪宋',32.0,'2020-10-11','新能源')

批量删除

• 使用 in 删除

/**
* 通过foreach完成批量删除
* @param ids
* @return
*/
int deleteBatchByForeach(@Param("ids") Long[] ids);

<!--
	collection：集合或数组
	item：集合或数组中的元素
	separator：分隔符
	open：foreach 标签中所有内容的开始
	close：foreach 标签中所有内容的结束
-->
<delete id="deleteBatchByForeach">
  delete from t_car where id in
  <foreach collection="ids" item="id" separator="," open="(" close=")">
    #{id}
  </foreach>
</delete>

@Test
public void testDeleteBatchByForeach(){
    CarMapper mapper = SqlSessionUtil.openSession().getMapper(CarMapper.class);
    int count = mapper.deleteBatchByForeach(new Long[]{40L, 41L, 42L});
    System.out.println("删除了几条记录：" + count);
    SqlSessionUtil.openSession().commit();
}

• 使用 or 删除

/**
* 通过foreach完成批量删除
* @param ids
* @return
*/
int deleteBatchByForeach2(@Param("ids") Long[] ids);

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

@Test
public void testDeleteBatchByForeach2(){
    CarMapper mapper = SqlSessionUtil.openSession().getMapper(CarMapper.class);
    int count = mapper.deleteBatchByForeach2(new Long[]{40L, 41L, 42L});
    System.out.println("删除了几条记录：" + count);
    SqlSessionUtil.openSession().commit();
}

批量添加  

/**
* 批量添加，使用foreach标签
* @param cars
* @return
*/
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>

@Test
public void testInsertBatchByForeach(){
    CarMapper mapper = SqlSessionUtil.openSession().getMapper(CarMapper.class);
    Car car1 = new Car(null, "2001", "兰博基尼", 100.0, "1998-10-11", "燃油车");
    Car car2 = new Car(null, "2001", "兰博基尼", 100.0, "1998-10-11", "燃油车");
    Car car3 = new Car(null, "2001", "兰博基尼", 100.0, "1998-10-11", "燃油车");
    List<Car> cars = Arrays.asList(car1, car2, car3);
    int count = mapper.insertBatchByForeach(cars);
    System.out.println("插入了几条记录" + count);
    SqlSessionUtil.openSession().commit();
}

七、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_time produceTime,car_type carType</sql>

<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>

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