动态SQL是MyBatis最强大的特性之一。用于实现动态SQL的主要元素如下：
1、if
2、choose、when、otherwise
3、trim、where、set
4、foreach
EmpMapper.xml配置

<select id="getEmpByIf" resultType="Emp" parameterType="Emp">
    select * from emp where 1 = 1
    <if test="job != null and job != ''">
        and job = #{
    
    job}
    </if>
    <if test="deptno != null ">
        and deptno = #{
    
    deptno}
    </if>
</select>

2、choose、when、otherwise
类似于Java中的switch case default

<select id="getEmpByChoose" resultType="Emp" parameterType="Emp">
    select * from emp where 1 = 1
    <choose>
        <when test="job != null">
            and job = #{
    
    job}
        </when>
        <when test="deptno != null">
            and deptno = #{
    
    deptno}
        </when>
        <otherwise>
            and mgr = #{
    
    mgr}
        </otherwise>
    </choose>
</select>

where

<select id="getEmpByWhere" resultType="Emp" parameterType="Emp">
    select * from emp
    <where>
        <if test="job != null and job != ''">
            and job = #{
    
    job}
        </if>
        <if test="deptno != null">
            and deptno = #{
    
    deptno}
        </if>
    </where>
</select>

set

<update id="updateEmpBySet" parameterType="Emp">
    update emp
    <set>
        <if test="ename != null and ename != ''">
            ename = #{
    
    ename},
        </if>
        <if test="job != null and job != ''">
            job = #{
    
    job},
        </if>
    </set>
    where empno = #{
    
    empno}
</update>