动态sql详解

一:where和if标签;
<select id="list" parameterType="AddressBook" resultType="AddressBook">
        select * from address_book
        <where>
            <if test="userId != null">
                and user_id = #{userId}
            </if>
            <if test="phone != null">
                and phone = #{phone}
            </if>
            <if test="isDefault != null">
                and is_default = #{isDefault}
            </if>
        </where>
    </select>
	
	1.where标签会动态识别其中语句中的and,or关键字并且自动去除条件起前面多余的and和or,如果where内部的if都不成立,甚至连where自身都不会拼接;
	2.if标签用于判断,成立则拼接内部语句,否之则不拼接;
	
二:foreach标签;
<select id="getSetmealIdsByDishIds" resultType="java.lang.Long">
        select setmeal_id from setmeal_dish where dish_id in
        <foreach collection="dishIds" item="dishId" separator="," open="(" close=")">
            #{dishId}
        </foreach>
    </select>
	
	1.foreach collection表示集合的名字,item表示集合的元素,separator表示分隔符,open,close表示开头和结尾的符号;
	2.形成的sql语句就类似于 select setmeal_id from setmeal_dish where dish_id in (1,2,3);
	
	
	
三:set标签;
<update id="update">
        update orders
        <set>
            <if test="status != null">
                status = #{status},
            </if>
            <if test="payStatus != null">
                pay_status = #{payStatus},
            </if>
            <if test="deliveryStatus != null">
                delivery_status = #{deliveryStatus},
            </if>
            <if test="cancelReason != null">
                cancel_reason = #{cancelReason},
            </if>
            <if test="cancelTime != null">
                cancel_time = #{cancelTime},
            </if>
        </set>
    </update>
	
	1.用于替代update语句中的set关键字,并且能动态去除多余的,  ;
	
四:sql include标签:
略.....;