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>
