Mybatis之动态SQL

1.if标签

根据标签中的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

    <!-- List<Emp> getEmpByDyCondition(Emp emp); -->
    <select id="getEmpByDyConditionOne" resultType="Emp">
        select * from t_emp where 1 = 1
        <if test="empName != null and empName != ''">
            and emp_name = #{empName}
        </if>
        <if test="age != null and age != ''">
            and age = #{age}
        </if>
        <if test="sex != null and sex != ''">
            and sex = #{sex}
        </if>
        <if test="email != null and email != ''">
            and email = #{email}
        </if>
    </select>

2.where标签
* 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
* 当where标签中没有内容时，此时where标签没有任何内容
* 注意：where标签不能将其中内容后面多余的and符号去掉，and只能放在前面

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

    </select>

3.trim标签
* prefix/suffix：在trim标签中内容前面或后面添加指定内容
* prefixOverrides/suffixOverrides: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

    <select id="getEmpByDyCondition" resultType="Emp">
        select <include refid="empColumns" /> from t_emp
        <trim prefix="where" suffixOverrides="and|or" prefixOverrides="and|or">
            <if test="empName != null and empName != ''">
                and emp_name = #{empName}
            </if>
            <if test="age != null and age != ''">
                and age = #{age}
            </if>
            <if test="sex != null and sex != ''">
                    and sex = #{sex}
            </if>
            <if test="sex = null and sex = ''">
                and sex = '男'
            </if>
            <if test="email != null and email != ''">
                and email = #{email}
            </if>
        </trim>
    </select>

4.choose、when、otherwise标签

相当于if...else if...else

* when至少要有一个，otherwise最多只有一个

    <!--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 != ''">
                    and age = #{age}
                </when>
                <when test="sex != null and sex != ''">
                    and sex = #{sex}
                </when>
                <when test="email != null and email != ''">
                    and email = #{email}
                </when>
                <otherwise> did = 1 </otherwise>
            </choose>
        </where>
    </select>

5.foreach标签
* 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条数据
* 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

    <!--int deleteMoreByarray(Integer[] eids);-->
    <delete id="deleteMoreByarray">
        <!--方法1
        delete from t_emp where eid in
        <foreach collection="eids" item="eid" separator="," open="(" close=")">
            #{eid}
        </foreach>
        -->
        <!--方法二-->
        delete from t_emp where
        <foreach collection="eids" item="eid" separator="or">
            eid = #{eid}
        </foreach>
    </delete>

    <!--int insertMoreByList(@Param("emps") List<Emp> emps);-->
    <insert id="insertMoreByList">
        insert into t_emp(emp_name,password,age,sex,email) values
        <foreach collection="emps" item="emp" separator=",">
            (#{emp.empName},#{emp.password},#{emp.age},#{emp.sex},#{emp.email})
        </foreach>
    </insert>

6.sql标签
* 设置sql片段：<sql id="empColumns">eid,emp_name,age,sex,email</sql>
* 引用sql片段：select <include refid="empColumns" /> from t_emp

 <sql id="empColumns">eid,emp_name,age,sex,email</sql>
 <select id="getEmpByDyCondition" resultType="Emp">
    select <include refid="empColumns" /> from t_emp
 </select>