数据库遇到的问题

　　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。

5.

SELECT * from m_material_sales_price where material_id = 406000003 and package_unit_id = null
SELECT * from m_material_sales_price where material_id = 406000003 and package_unit_id is null

上一条SQL查询结果为0条，下一条SQL查询结果为1。注意用关键字is。

　　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！