mybatis中的动态sql问题

1、if（常用）

if：根据标签中test属性所对应的表达式决定标签中的内容是否需要拼接到SQL中

/**
     * 多条件查询
     */
    List<Emp> getEmpByCondition(Emp emp);

当empName为null和“ ”时，会拼接and age，此时会报错，可以1=1恒成立解决。

用and表示&&（并且）
emp_name是字段名
if只有test标签且必须使用

<!--List<Emp> getEmpListByMoreTJ(Emp emp);-->
    <select id="getEmpListByMoreTJ" resultType="Emp">
select * from t_emp where 1=1
<if test="empName!= '' and empName!= null">
    and emp_name = #{empName}
</if>
<if test="age != '' and age != null">
    and age = #{age}
</if>
<if test="sex != '' and sex != null">
    and sex = #{sex}
</if>
</select>
@Test
    public void testGetEmpByCondition(){
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        DynamicSQLMapper mapper = sqlSession.getMapper(DynamicSQLMapper.class);
        List<Emp> list = mapper.getEmpByCondition(new Emp(null, "陈智", 33, "女", null));
        System.out.println(list);
    }
select eid,emp_name,age,sex,email from t_emp where emp_name = ? and age = ? or sex = ?

2、where

当where标签中有内容时，会自动生成where关键字，并且将内容前多余的and或or去掉（在程序执行后生成的sql会将内容前多余的and或or去掉）

and写在第二句是为了和上面拼接，写在第一句是和下面拼接（即固定有elect * from t_emp emp_name= ?）

<select id="getEmpListByMoreTJ2" resultType="Emp">
    select * from t_emp
<where>
<if test="empName != '' and empName != null">
    emp_name = #{empName }
</if>
<if test="age != '' and age != null">
    and age = #{age}
</if>
<if test="sex != '' and sex != null">
    and sex = #{sex}
</if>
</where>
</select>

当where标签中没有内容时(或者使用getEmpByCondition传入的值全为null或“ ”)，此时where标签没有任何效果。则直接SQL语句为select * from t_emp

--------------------分割--------------------

where标签不能将其中内容后面多余的and或or去掉：（会报错）

语句为select * from t_emp emp_name= ? and

<select id="getEmpListByMoreTJ2" resultType="Emp">
    select * from t_emp
<where>
<if test="empName != '' and empName != null">
    emp_name = #{empName } and
</if>
<if test="age != '' and age != null">
    age = #{age} and
</if>
<if test="sex != '' and sex != null">
    sex = #{sex}
</if>
</where>
</select>

3、trim

where的增强

若标签中有内容时：

prefix|suffix：将trim标签中内容前面或后面添加指定内容
suffixOverrides|prefixOverrides：将trim标签中内容前面或后面去掉指定内容

若标签中没有内容时，trim标签也没有任何效果（跟上面的where一样）

<!--List<Emp> getEmpByCondition(Emp emp);-->
    <select id="getEmpByCondition" resultType="Emp">
        select <include refid="empColumns"></include> from t_emp
        <trim prefix="where" suffixOverrides="and|or">
            <if test="empName != null and empName != ''">
                emp_name = #{empName} and
            </if>
            <if test="age != null and age != ''">
                age = #{age} or
            </if>
            <if test="sex != null and sex != ''">
                sex = #{sex} and
            </if>
            <if test="email != null and email != ''">
                email = #{email}
            </if>
        </trim>
    </select>

4.choose、when、otherwise

相当于if...else if...else

/**
     * 测试choose、when、otherwise
     */
    List<Emp> getEmpByChoose(Emp emp);

when至少要有一个，otherwise最多只能有一个

与第一点if的区别：choose只会满足一个条件便退出，一个不满足则执行otherwise

<!--List<Emp> getEmpByChoose(Emp emp);-->
    <select id="getEmpByChoose" resultType="Emp">
        select * from t_emp
        <where>
            <choose>
                <when test="empName != null and empName != ''">
                    emp_name = #{empName}
                </when>
                <when test="age != null and age != ''">
                    age = #{age}
                </when>
                <when test="sex != null and sex != ''">
                    sex = #{sex}
                </when>
                <when test="email != null and email != ''">
                    email = #{email}
                </when>
                <otherwise>
                    did = 1
                </otherwise>
            </choose>
        </where>
    </select>
@Test
    public void testGetEmpByChoose(){
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        DynamicSQLMapper mapper = sqlSession.getMapper(DynamicSQLMapper.class);
        List<Emp> list = mapper.getEmpByChoose(new Emp(null, "", null, "", ""));
        System.out.println(list);
    }

5、foreach

collection:设置需要循环的数组或集合
item:表示数组或集合中的每一个数据
separator:循环体之间的分割符
open:foreach标签所循环的所有内容的开始符
close:foreach标签所循环的所有内容的结束符

5.1批量删除

/**
     * 通过数组实现批量删除
     */
    int deleteMoreByArray(@Param("eids") Integer[] eids);
<!--int deleteMoreByArray(@Param("eids") Integer[] eids);-->
    <delete id="deleteMoreByArray">
        delete from t_emp where
        <foreach collection="eids" item="eid" separator="or">
            eid = #{eid}
        </foreach>
        <!--
            delete from t_emp where eid in
            <foreach collection="eids" item="eid" separator="," open="(" close=")">
                #{eid}
            </foreach>
        -->
    </delete>
@Test
    public void testDeleteMoreByArray(){
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        DynamicSQLMapper mapper = sqlSession.getMapper(DynamicSQLMapper.class);
        int result = mapper.deleteMoreByArray(new Integer[]{12,13,14,15});
        System.out.println(result);
    }

5.2批量添加

/**
     * 通过list集合实现批量添加
     */
    int insertMoreByList(@Param("emps") List<Emp> emps);
<!--int insertMoreByList(@Param("emps") List<Emp> emps);-->
    <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>

Arrays.asList该方法是将数组转化成List集合的方法

如果你的List只是用来遍历，就用Arrays.asList()。

如果你的List还要添加或删除元素，还是乖乖地new一个java.util.ArrayList，然后一个一个的添加元素。

@Test
    public void testInsertMoreByList(){
        SqlSession sqlSession = SqlSessionUtils.getSqlSession();
        DynamicSQLMapper mapper = sqlSession.getMapper(DynamicSQLMapper.class);
        Emp emp1 = new Emp(null,"a1",23,"男","123@qq.com");
        Emp emp2 = new Emp(null,"a2",23,"男","123@qq.com");
        Emp emp3 = new Emp(null,"a3",23,"男","123@qq.com");
        List<Emp> emps = Arrays.asList(emp1, emp2, emp3);
        System.out.println(mapper.insertMoreByList(emps));
    }

6、sql标签

设置SQL片段：<sql id="empColumns">eid,emp_name,age,sex,email</sql>

引用SQL片段：<include refid="empColumns"></include>