【MyBatis】四、MyBatis中的动态SQL标签

动态SQL

动态SQL语句是动态的拼接Mybatis中SQL语句的情况，可以动态的在Mybatis中使用SQL

if语句

if语句的xml文件：

    <select id="getEmpByCondition" resultType="Emp">
        select * from t_emp where 1 = 1
        <if test="empName != null and empName != ''">
            emp_name = #{empName}
        if>
        <if test="sex != null and sex != ''">
            and sex = #{sex}
        if>
        <if test="age != null and age != ''">
            and age = #{age}
        if>
        <if test="email != null and email != ''">
            and email = #{email}
        if>
    select>
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传入对象来进行调用：

    @Test
    public void test() {
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        DynamicSQLMapper mapper = sqlSession.getMapper(DynamicSQLMapper.class);
        List<Emp> empList = mapper.getEmpByCondition(new Emp(null, "", 23, "男", "123@qq.com", null));
        System.out.println(empList);
    }
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WHERE语句

where标签中的and会被自动去掉，并且若没有合适的内容，则不会添加where关键字

注意：where标签只能去掉条件前的and、五福去掉条件后的and

    <select id="getEmpByCondition" resultType="Emp">
        select * from t_emp
        <where>
            <if test="empName != null and empName != ''">
                emp_name = #{empName}
            if>
            <if test="sex != null and sex != ''">
                and sex = #{sex}
            if>
            <if test="age != null and age != ''">
                and age = #{age}
            if>
            <if test="email != null and email != ''">
                and email = #{email}
            if>
        where>
    select>
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trim标签

trim标签会在其内容中没有合适的内容时不添加其内容，若trim标签内有内容时，则会添加这个内容

    <select id="getEmpByCondition" resultType="Emp">
        select * from t_emp
        <trim prefix="where" suffixOverrides="and|or">
            <if test="empName != null and empName != ''">
                emp_name = #{empName} and
            if>
            <if test="sex != null and sex != ''">
                sex = #{sex} or
            if>
            <if test="age != null and age != ''">
                age = #{age} and
            if>
            <if test="email != null and email != ''">
                email = #{email}
            if>
        trim>
    select>
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choose、when、otherwise

choose是when和otherwise的父标签，when和otherwise要写在choose中，相当于编程语言中的if…else…if…else…

    <select id="getEmpByChoose" resultType="Emp">
        select * from t_emp
        <where>
            <choose>
                <when test="empName != null and empName != ''">
                    emp_name = #{empName}
                when>
                 <when test="sex != null and sex != ''">
                     sex = #{sex}
                 when>
                <otherwise>
                    did = 1
                otherwise>
            choose>
        where>
    select>
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注意这里when至少有一个、otherwise至多有一个，也可以没有

foreach

通过数组实现批量删除

mapper接口：

    /**
     * 通过数组实现批量删除（形参传入数组）
     */
    int deleteMoreByArray(@Param("eids") Integer[] eids);
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xml：

    <delete id="deleteMoreByArray">
        DELETE from t_emp where eid in
            <foreach collection="eids" item="eid" separator="," open="(" close=")">
                #{eid}
            foreach>
    delete>
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xml的第二种写法：

    <delete id="deleteMoreByArray">
        delete from t_emp where
        <foreach collection="eids" item="eid" separator="or">
            eid = #{eid}
        foreach>
    delete>
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通过集合实现批量添加

建立接口：

    /**
     * 通过list实现批量添加
     */
    int insertMoreByList(@Param("emps") List<Emp> list);
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建立xml：

    <insert id="insertMoreByList">
        insert into t_emp values
        <foreach collection="emps" item="emp" separator=",">
            (null, #{emp.empName}, #{emp.age}, #{emp.sex}, #{emp.email}, null)
        foreach>
    insert>
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sql标签

sql标签用来将常用的一些字段记录，下次在有这些字段出现时可以简易的替换这些字段：

使用include标签来引用定义的sql片段

    <sql id="empColumn">eid, emp_name, age, sex, emailsql>
    
    <select id="getEmpByConditionOne" resultType="Emp">
        select <include refid="empColumn">include> from t_emp
    select>
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