mybatis学习之动态sql

mybatis的动态sql语句很强大，在mapper映射文件中使用简单的标签即可实现该效果，下面一个个记录：

1、select查询

简单的select类似如下：

<select id="findById" resultMap="StudentResult" parameterType="Integer">
    select * from t_student where id = #{id}
</select>

1）if（常用于各种查询的条件判断部分）

<select id="searchStudents" parameterType="Map" resultMap="StudentResult">
    select * from t_student
    where gradeId = #{gradeId}
    <if test="name != null">
        and name like #{name}
    </if>
    <if test="age != null">
        and age = #{age}
    </if>
</select>

结合where标签使用如下：

<select id="searchStudents3" parameterType="Map" resultMap="StudentResult">
    select * from t_student
    <where>
        <if test="gradeId != null">
            gradeId = #{gradeId}
        </if>
        <if test="name != null">
            and name like #{name}
        </if>
        <if test="age != null">
            and age = #{age}
    </if>
    </where>
</select>

2）choose（同if..else..类似）

<select id="searchStudents2" parameterType="Map" resultMap="StudentResult">
    select * from t_student
    <choose>
        <when test="searchBy=='gradeId'">
            where gradeId = #{gradeId}
        </when>
        <when test="searchBy=='name'">
            where name like #{name}
        </when>
        <otherwise>
            where age = #{age}
        </otherwise>
    </choose>
</select>

3）trim

<select id="searchStudents4" parameterType="Map" resultMap="StudentResult">
    select * from t_student
    <trim prefix="where" prefixOverrides="and|or">
        <if test="gradeId != null">
            gradeId = #{gradeId}
        </if>
        <if test="name != null">
            and name like #{name}
        </if>
        <if test="age != null">
            and age = #{age}
        </if>
    </trim>
</select>

prefix前置，prefixOverrides前置覆盖，简单理解为：trim子句中最前面的and或者or用where替换。

4）foreach

<select id="searchStudents5" parameterType="Map" resultMap="StudentResult">
    select * from t_student
    <if test="gradeIds != null">
        <where>
            gradeId in
            <foreach collection="gradeIds" item="gradeId" open="("
                close=")" separator=",">
                #{gradeId}
            </foreach>
        </where>
    </if>
</select>

collections即数组集合，可以是list类型，如arrayList等，open指定左侧拼接方式，close指定右侧，separator指定分隔符，这里拼接后为括号括起来逗号隔开的字符串，从而用于in查询。

2、update

<update id="updateStudent" parameterType="Student">
    update t_student
    <set>
        <if test="name != null">
            name = #{name},
        </if>
        <if test="age != null">
            age = #{age}        <!-- 自动剔除最后的逗号 -->
        </if>
    </set>
    where id = #{id}
</update>

<update id="update" parameterType="Student">
    update t_student set name =
    #{name},age = #{age} where id = #{id}
</update>

3、delete

<delete id="delete" parameterType="Integer">
    delete from t_student where id = #{id}
</delete>

4、insert

<!-- 插入 -->
<insert id="add" parameterType="Student">
    insert into t_student(id,name,age) values(null,#{name},#{age})
</insert>