MyBatis--3 (dynamic SQL related knowledge)

1. Dynamic SQL

• SQL statements that change with user input or changes in external conditions are called dynamic SQL .
• mybatisThe dynamic queries in are all composed of multiple tags

2. <if>, <where>and<set>

1. <if>: Used to determine whether the condition is established. Use testattributes for conditional judgment, and if the condition is true, then splicing SQL .

• example:

<if test="name != null">
   name like concat('%',#{name},'%')
</if>

• Example:

<select id="list" resultType="com.itheima.pojo.Emp">
        select *
        from emp where 
        <if test="name!=null">
            name like concat('%', #{name}, '%')
        </if>
        <if test="gender!=null">
            and gender = #{gender}
        </if>
        <if test="begin!=null and end!=null">
            and entrydate between #{begin} and #{end}
        </if>
        order by update_time desc
    </select>

• However, the above code has a shortcoming, that is, keywords such as where, andand can not be dynamically generated , which will cause grammatical errorsor

2. <where>: The where element will only insert the where clause when the child element has content . ANDAnd the or at the beginning of the clause will be automatically removed OR.

• Improve:

   select *
        from emp
        <where>
            <!-- where的作用：1.动态的生成where  2.自动判断是否需要and , or等关键字 -->
        <if test="name!=null">
            name like concat('%', #{name}, '%')
        </if>
        <if test="gender!=null">
            and gender = #{gender}
        </if>
        <if test="begin!=null and end!=null">
            and entrydate between #{begin} and #{end}
        </if>
        </where>
        order by update_time desc
    </select>

3. <set>Replaces keywords and strips commas (used in statements) setbetween multiple assignments .update

• Example in xml file:

    <update id="update2">
        update emp
        <set>
            <if test="username!=null">username = #{username},</if>
            <if test="name!=null">name = #{name},</if>
            <if test="gender !=null">gender = #{gender },</if>
            <if test="image!=null">image = #{image},</if>
            <if test="job!=null">job = #{job},</if>
            <if test="entryDate!=null">entrydate = #{entryDate},</if>
            <if test="deptId!=null">dept_id = #{deptId},</if>
            <if test="updateTime!=null">update_time = #{updateTime}</if>
        </set>
           where id = #{id}
    </update>

3.<foreach>

1. SQL statement

delete from emp where id in (1,2,3);

2. interface method

//批量删除
public void deleteBylds(List<Integer> ids);

3. XML mapping file

  <!--批量删除员工的操作 (18,19, 20)-->
    <!--
        collection :遍历的集合,与mapper中的集合名保持一致即可
        item:遍历出来的元素,名称任取
        separator:分隔符
        open:遍历开始前拼接的SQ工片段
        close:遍历结束局拼接的SQL片段
    -->
    <delete id="deleteByIds">
        delete from emp where id in
        <foreach collection="ids" item="id" open="(" close=")" separator=",">
            #{id}
        </foreach>
    </delete>

4.<sql>,<include>

1. <sql><include>These two tags solve the problem of code reusability, similar to the definition and call of methods

• Original select statement (comparison)

<select id="list" resultType="com.itheima.pojo.Emp">
    select id,username, password,name, gender,image,job,entrydate,dept_id,create_time,update_time
    from emp
        <where>
            <if test="name!=null">
                name like concat('%', #{name}, '%')
            </if>
            <if test="gender!=null">
                and gender = #{gender}
            </if>
            <if test="begin!=null and end!=null">
                and entrydate between #{begin} and #{end}
            </if>
        </where>
    order by update_time desc
</select>

2. <sql>Extract a large number of multiplexed SQL statements

 <sql id="commonSelect">
 <!-- id属性是取得唯一标识名 -->
        select id,username, password,name, gender,image,job,entrydate,dept_id,create_time,update_time
		from emp
    </sql>

3. <include>use reuse code

<select id="list" resultType="com.itheima.pojo.Emp">
    <include refid="commonSelect"/>
        <where>
            <if test="name!=null">
                name like concat('%', #{name}, '%')
            </if>
            <if test="gender!=null">
                and gender = #{gender}
            </if>
            <if test="begin!=null and end!=null">
                and entrydate between #{begin} and #{end}
            </if>
        </where>
        order by update_time desc
</select>