标签：tail false isequal 左连接 item limit 数据库概念 foreach 动态
最近在用mybatis写sql时候，遇到了一些问题。在以下sql中：
<select id="selectByCondition" parameterType="map" resultMap="BaseResultMap"> select count(*) purchase_num, b.*, a.create_time AS s_create_time, a.sales_order_no, a.amount_tax_sum, a.sales_order_status_name, a.marker_date, a.checker_date,a.sales_order_id from b_sales_order a, b_sales_order_detail b WHERE a.sales_order_id = b.sales_order_id AND a.delete_flag =‘0‘ <if test="map.statusList !=null and map.statusList.size > 0"> AND a.sales_order_status_code in <foreach item="item" index="index" collection="map.statusList" open="(" separator="," close=")"> #{item} </foreach> </if> <if test="map.flag == true and map.isEqual ==true"> AND b.request_number = b.outstock_number </if> <if test="map.flag == true and map.isEqual ==false"> AND b.request_number > b.outstock_number </if> <if test="map.markerId!=null"> AND a.marker_id=#{map.markerId} </if> /*GROUP BY a.sales_order_id*/ ORDER BY a.marker_date DESC LIMIT #{map.offset},#{map.pageSize} </select>
1. 除开动态sql，即使我用
select count(*) purchase_num, b.*, a.create_time AS s_create_time, a.sales_order_no, a.amount_tax_sum, a.sales_order_status_name, a.marker_date, a.checker_date,a.sales_order_id from b_sales_order a, b_sales_order_detail b WHERE a.sales_order_id = b.sales_order_id AND a.delete_flag =‘0‘
查询结果也只有1条记录。当我把 count(*)删除时，开始有多条数据。这里count是统计函数，由于使用了它，数据只有1行。
2.我用
select b.*, a.create_time AS s_create_time, a.sales_order_no, a.amount_tax_sum, a.sales_order_status_name, a.marker_date, a.checker_date,a.sales_order_id from b_sales_order a, b_sales_order_detail b WHERE a.sales_order_id = b.sales_order_id AND a.delete_flag =‘0‘ AND marker_id = 11 GROUP BY a.sales_order_id
查询时候，发现数据也只有1条。我这里用到了分组查询。当我把
GROUP BY a.sales_order_id删除后，开始有多条数据。
3.在以下SQL中
<select id="countByCondition" parameterType="map" resultType="java.lang.Integer"> select COUNT(*) from (select COUNT(*) from b_sales_order a, b_sales_order_detail b WHERE a.sales_order_id = b.sales_order_id AND a.delete_flag =‘0‘ <if test="map.statusList !=null and map.statusList.size > 0"> AND a.sales_order_status_code in <foreach item="item" index="index" collection="map.statusList" open="(" separator="," close=")"> #{item} </foreach> </if> <if test="map.flag == true and map.isEqual ==true"> AND b.request_number = b.outstock_number </if> <if test="map.flag == true and map.isEqual ==false"> AND b.request_number > b.outstock_number </if> <if test="map.markerId!=null"> AND a.marker_id=#{map.markerId} </if> GROUP BY a.sales_order_id) c
可以统计出订单个数。
4.
select b.*, a.create_time AS s_create_time, a.sales_order_no, a.amount_tax_sum, a.sales_order_status_name, a.marker_date, a.checker_date,a.sales_order_id from b_sales_order a LEFT JOIN b_sales_order_detail b on a.sales_order_id = b.sales_order_id AND a.delete_flag =‘0‘ AND marker_id = 11
当我用左连接时候，发现订单表的数据全都在，当详情表的数据匹配不上时候，详情表所有字段为空。
极端情况，订单表一条纪录，详情表对应13条，那么进行左连接时候，发现有13条记录！在我们需要按详情表的字段作为搜索条件，查询结果为订单表时，
订单结果就重复了12条！因此，左连接不能乱用。
综上，以上问题都是在实际项目中遇到的，也反映出对数据库知识的欠缺。要对 左连接、where、分组、统计、笛卡尔积这些数据库概念有清晰认识！
标签：tail false isequal 左连接 item limit 数据库概念 foreach 动态
原文地址：http://www.cnblogs.com/parkdifferent/p/7382630.html
