Mybatis动态SQL

一、if标签

需求：多条件查询。可能的条件包括：品牌（brand）、指导价格（guide_price）、汽车类型（car_type）

CarMapper接口

import java.util.List;public interface CarMapper {
/
* 根据多条件查询Car
* @param brand
* @param guidePrice
* @param carType
* @return
*/
List<Car> selectByMultiCondition(@Param("brand") String brand, @Param("guidePrice") Double guidePrice, @Param("carType") String carType);
}

CarMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"><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 != ''">andbrand 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>

测试程序

import org.junit.Test;
import java.util.List;public class CarMapperTest {@Testpublic void testSelectByMultiCondition(){CarMapper mapper = SqlSessionUtil.openSession().getMapper(CarMapper.class);List<Car> cars = mapper.selectByMultiCondition("丰田", 20.0, "燃油车");System.out.println(cars);}
}

执行结果：

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

继续使用if标签中的需求

CarMapper接口

/
* 根据多条件查询Car，使用where标签
* @param brand
* @param guidePrice
* @param carType
* @return
*/
List<Car> selectByMultiConditionWithWhere(@Param("brand") String brand, @Param("guidePrice") Double guidePrice, @Param("carType") String carType);

CarMapper.xml

<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>

测试程序

@Test
public void testSelectByMultiConditionWithWhere(){CarMapper mapper = SqlSessionUtil.openSession().getMapper(CarMapper.class);List<Car> cars = mapper.selectByMultiConditionWithWhere("丰田", 20.0, "燃油车");System.out.println(cars);
}

运行结果：

如果所有条件都是空呢？

它可以自动去掉前面多余的and，那可以自动去掉后面多余的and吗？

<select id="selectByMultiConditionWithWhere" resultType="car">select * from t_car<where><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></where>
</select>

测试程序

// 让最后一个条件为空
List<Car> cars = mapper.selectByMultiConditionWithWhere("丰田", 20.0, "");

运行结果：

很显然，后面多余的and是不会被去除的

三、trim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）

CarMapper接口

/
* 根据多条件查询Car，使用trim标签
* @param brand
* @param guidePrice
* @param carType
* @return
*/
List<Car> selectByMultiConditionWithTrim(@Param("brand") String brand, @Param("guidePrice") Double guidePrice, @Param("carType") String carType);

CarMapper.xml

<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>

测试程序

@Test
public void testSelectByMultiConditionWithTrim(){CarMapper mapper = SqlSessionUtil.openSession().getMapper(CarMapper.class);List<Car> cars = mapper.selectByMultiConditionWithTrim("丰田", 20.0, "");System.out.println(cars);
}

如果所有条件为空，where会被加上吗？答案是不会的。

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“,”
比如我们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。

CarMapper接口

/
* 更新信息，使用set标签
* @param car
* @return
*/
int updateWithSet(Car car);

CarMapper.xml

<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>

测试程序

@Test
public void testUpdateWithSet(){CarMapper mapper = SqlSessionUtil.openSession().getMapper(CarMapper.class);Car car = new Car(38L,"1001","丰田霸道2",10.0,"",null);int count = mapper.updateWithSet(car);System.out.println(count);SqlSessionUtil.openSession().commit();
}

执行结果：

五、choose when otherwise

这三个标签是在一起使用的：

<choose><when></when> //else if<when></when><when></when><otherwise></otherwise> //else
</choose>//等同于：
if(){}else if(){}else if(){}else if(){}else{}

只有一个分支会被选择！！！！

需求：先根据品牌查询，如果没有提供品牌，再根据指导价格查询，如果没有提供指导价格，就根据生产日期查询。

CarMapper接口

/
* 使用choose when otherwise标签查询
* @param brand
* @param guidePrice
* @param produceTime
* @return
*/
List<Car> selectWithChoose(@Param("brand") String brand, @Param("guidePrice") Double guidePrice, @Param("produceTime") String produceTime);

CarMapper.xml

<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>

测试程序

@Test
public void testSelectWithChoose(){CarMapper mapper = SqlSessionUtil.openSession().getMapper(CarMapper.class);//List<Car> cars = mapper.selectWithChoose("丰田霸道", 20.0, "2000-10-10");//List<Car> cars = mapper.selectWithChoose("", 20.0, "2000-10-10");//List<Car> cars = mapper.selectWithChoose("", null, "2000-10-10");List<Car> cars = mapper.selectWithChoose("", null, "");System.out.println(cars);
}

六、foreach标签

循环数组或集合，动态生成sql，比如这样的SQL：

-- 批量删除
delete from t_car where id in(1,2,3);
delete from t_car where id = 1 or id = 2 or id = 3;-- 批量添加
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（in）

CarMapper接口

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

CarMapper.xml

<!--
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>

测试程序

@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();
}

执行结果：

• 批量删除（or）

CarMapper接口

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

CarMapper.xml

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

测试程序

@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();
}

执行结果：

• 批量添加

CarMapper接口

/
* 批量添加，使用foreach标签
* @param cars
* @return
*/
int insertBatchByForeach(@Param("cars") List<Car> cars);

CarMapper.xml

<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>

测试程序

@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();
}

执行结果：

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>