MtBatis学习笔记之动态SQL

if标签

1、if标签中的test属性是必须的
2、if标签中test属性的值是false或者true
3、如果test是true，则if标签中的sql语句就会拼接，反之，则不会拼接
4、test属性中可以使用的是：
当使用了@Param注解，那么test中要出现的是@Param注解指定的参数名，@Param（“brand”），那么这里只能使用brand
当没有使用@Param注解，那么test中要出现的是：param1，param2，arg0，arg1…
当使用了POJO，那么test中出现的是POJO类的属性名。
5、在mybatis的动态SQL当中，不能使用&&，只能使用and。

//CarMapper.java
 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>

//CarMapperTest.java
    @Test
    public void testSelectById(){
        SqlSession sqlSession = SqlSessionUtil.openSession();
        CarMapper mapper = sqlSession.getMapper(CarMapper.class);
        List<Car> cars = mapper.selectByMultiCondition("比亚迪",2.0,"新能源");
        cars.forEach(car -> System.out.println(car));
        sqlSession.close();
    }

↑但是上面写的SQL语句明显存在着逻辑的问题，如果第一个判断语句为false，里面的SQL语句没有执行而后面的加上了，就存在SQL语句的语法错误

所以改为下面这种语句

    <select id="selectByMultiCondition" resultType="Car">
        select * from t_car where 1 = 1
        <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>

where标签

where标签的作用：让where子句更加多态智能

• 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

wher标签是专门负责where子句动态生成的

    <select id="selectByMultiConditionWhere" 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>

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）

    <select id="selectByMultiConditionTrim" 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>

• prefix="where"是在trim标签所有内容的前面添加where
• suffixOverrides="and|or"把trim标签中内容的后缀and或or去掉

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

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 = #{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>

    @Test
    public void testBySet(){
        SqlSession sqlSession = SqlSessionUtil.openSession();
        CarMapper mapper = sqlSession.getMapper(CarMapper.class);
        Car car = new Car(5L,null,null,"丰田sucker",null,null);
        mapper.updateWithSet(car);
        sqlSession.commit();
        sqlSession.close();
    }

choose when otherwise

这三个标签是在一起使用的
相当于if、elseif、else
只有一个分支会被选择

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

    <select id="selectByChoose" 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>
                    car_type = #{carType}
                otherwise>
            choose>
        where>
    select>

    @Test
    public void testchoose(){
        SqlSession sqlSession = SqlSessionUtil.openSession();
        CarMapper mapper = sqlSession.getMapper(CarMapper.class);
        List<Car> cars = mapper.selectByChoose("", 5.0,null);
        cars.forEach(car -> System.out.println(car));
        sqlSession.close();

    }

批量删除

    int deleteByIds(String ids[]);

foreach标签的属性：

• collection：指定数组或者集合
• item：代表数组或集合中的元素
• separator：循环之间的分隔符

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

    @Test
    public void testDelteAll(){
        SqlSession sqlSession = SqlSessionUtil.openSession();
        CarMapper mapper = sqlSession.getMapper(CarMapper.class);
        Long[] ids ={7L, 8L, 9L};
        int i = mapper.deleteByIds(ids);
        System.out.println(i);
        sqlSession.commit();
        sqlSession.close();
    }

批量插入

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

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

    @Test
    public void testInsertBatch(){
        SqlSession sqlSession = SqlSessionUtil.openSession();
        CarMapper mapper = sqlSession.getMapper(CarMapper.class);
        Car car1 = new Car(null,"八嘎亚路1",30.8,"燃油第",null,"111");
        Car car2 = new Car(null,"八嘎亚路2",30.8,"燃油第",null,"111");
        Car car3 = new Car(null,"八嘎亚路3",30.8,"燃油第",null,"111");
        List<Car> cars = new ArrayList<>();
        cars.add(car1);
        cars.add(car2);
        cars.add(car3);
        mapper.insertBatch(cars);
        sqlSession.commit();
        sqlSession.close();
    }

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="carColumnNameSql">
	id,
	car_num as carNum,
	brnd,
	guide_price as guidePrice,
	produce_time as produceTime,
	car_type as carType
sql>
<select id="...." resultType="car">
	select
		<include refid="carColumnNameSql">
	from t_car where id = #{id}
select>