Mybatis框架的动态SQL技术是一种根据特定条件动态拼装SQL语句的功能，它存在的意义是为了 解决 拼接SQL语句字符串时的痛点问题。
if
if标签可通过test属性的表达式进行判断，若表达式的结果为true，则标签中的内容会执行；反之 标签中的内容不会执行
<!--List<Emp> getEmpListByCondition(Emp emp);--> <select id="getEmpListByMoreTJ" resultType="Emp"> select * from t_emp where 1=1 <if test="ename != '' and ename != null"> and ename = #{ename} </if> <if test="age != '' and age != null"> and age = #{age} </if> <if test="sex != '' and sex != null"> and sex = #{sex} </if> </select>
where
where和if一般结合使用：
a>若where标签中的if条件都不满足，则where标签没有任何功能，即不会添加where关键字
b>若where标签中的if条件满足，则where标签会自动添加where关键字，并将条件最前方多余的
and去掉 注意：where标签不能去掉条件最后多余的and
<select id="getEmpListByMoreTJ2" resultType="Emp"> select * from t_emp <where> <if test="ename != '' and ename != null"> ename = #{ename} </if> <if test="age != '' and age != null"> and age = #{age} </if> <if test="sex != '' and sex != null"> and sex = #{sex} </if> </where> </select>
trim
trim用于去掉或添加标签中的内容
常用属性：
prefix：在trim标签中的内容的前面添加某些内容
prefixOverrides：在trim标签中的内容的前面去掉某些内容
suffix：在trim标签中的内容的后面添加某些内容
suffixOverrides：在trim标签中的内容的后面去掉某些内容
<select id="getEmpListByMoreTJ" resultType="Emp"> select * from t_emp <trim prefix="where" suffixOverrides="and"> <if test="ename != '' and ename != null"> ename = #{ename} and </if> <if test="age != '' and age != null"> age = #{age} and </if> <if test="sex != '' and sex != null"> sex = #{sex} </if> </trim> </select>
choose、when、otherwise
choose、when、 otherwise相当于if...else if..else
<!--List<Emp> getEmpListByChoose(Emp emp);--> <select id="getEmpListByChoose" resultType="Emp"> select <include refid="empColumns"></include> from t_emp <where> <choose> <when test="ename != '' and ename != null"> ename = #{ename} </when> <when test="age != '' and age != null"> age = #{age} </when> <when test="sex != '' and sex != null"> sex = #{sex} </when> <when test="email != '' and email != null"> email = #{email} </when> </choose> </where> </select>
foreach
<!--int insertMoreEmp(List<Emp> emps);--> <insert id="insertMoreEmp"> insert into t_emp values <foreach collection="emps" item="emp" separator=","> (null,#{emp.ename},#{emp.age},#{emp.sex},#{emp.email},null) </foreach> </insert> <!--int deleteMoreByArray(int[] eids);--> <delete id="deleteMoreByArray"> delete from t_emp where <foreach collection="eids" item="eid" separator="or"> eid = #{eid} </foreach> </delete> <!--int deleteMoreByArray(int[] eids);--> <delete id="deleteMoreByArray"> delete from t_emp where eid in <foreach collection="eids" item="eid" separator="," open="(" close=")"> #{eid} </foreach> </delete>
