昨天写了个对于我来说很复杂的sql语句。 这里先放个图：
现在的需要的数据如图：从product表中拿到id,name两个字段，从buy_contract_Item和sale_contract_Item中拿到quantity,totalMoney这两个字段，再从buy_contract和sale_contract中拿到contract_date字段，同时以合同签定时间为查询条件将商品各类的数量和金额sum,最后返回获取结果。表与表的关联关系如图。
后来问了下同事，拿到了返回结果：
一开始就犯了个错误，虽然从 buy_contract_Item和sale_contract_Item获取的信息比较多，但却不能以他们为主表，这是昨天半天还没有结果的主要原因。因为所有后续的这些表都是以商品为主，为商品的买进和卖出服务的。所以这里以商品为主表做查询。
第一步：从product表中拿到Id,Name两个字段（其他的字段在后面扩充）
SELECT id,name FROM product
第二步：从buy_contract_item中拿到productId,SUM(quantity),SUM(totalMoney)这两个字
段并按组分类
--productId 为了后面与product表做关联条件的
SELECT productId,SUM(quantity),SUM(totalMoney)
FROM buy_contract_item
GROUP BY productId
第三步：从sale_contract_item中拿到SUM(quantity),SUM(totalMoney)这两个字段并按组
分类
SELECT
productId,SUM(quantity),SUM(totalMoney)
FROM sale_contract_item
GROUP BY productId
第四步： 对第二步的SELECT语句进行扩充，联结buy_contract：
获取SUM(quantity),SUM(totalMoney)和contract_date
SELECT productId,SUM(quantity),SUM(totalMoney),contract_date
FROM buy_contract_item a
JOIN buy_contract b ON a.contract_id = b.id
GROUP BY a.product_id;
第五步：添加where查询条件，另外再加上别名
SELECT productId,SUM(quantity) AS buyQuantity,SUM(totalMoney) AS buyTotalMoney,b.contract_date
FROM buy_contract_item a
JOIN buy_contract b ON a.contract_id = b.id
WHERE 1=1 and (b.contract_date >= ‘2015-03-13‘ and b.contract_date <= ‘2015-03-15‘)
GROUP BY a.product_id;
第六步：写出sale的情况和上面类似
SELECT productId,SUM(quantity) AS saleQuantity,SUM(totalMoney) AS saleTotalMoney,d.contract_date
FROM buy_contract_item c
JOIN buy_contract d ON c.contract_id = d.id
WHERE 1=1 and (d.contract_date >= ‘2015-03-13‘ and d.contract_date <= ‘2015-03-15‘)
GROUP BY c.product_id;
第一步、第五步、第六步左联结
SELECT t.id,t.name,
IFNULL(buyQuantity,0) AS buyQuantity,
IFNULL(buyTotalMoney,0) AS buyTotalMoney,
IFNULL(saleQuantity,0) AS saleQuantity,
IFNULL(saleTotalMoney,0) AS saleTotalMoney
FROM product t
LEFT JOIN
(
SELECT productId,SUM(quantity) AS buyQuantity,SUM(totalMoney) AS buyTotalMoney,b.contract_date
FROM buy_contract_item a
JOIN buy_contract b ON a.contract_id = b.id
WHERE 1=1 and (b.contract_date >= ‘2015-03-13‘ and b.contract_date <= ‘2015-03-15‘)
GROUP BY a.product_id;
) m ON t.id = m.productId
LEFT JOIN
(
SELECT productId,SUM(quantity) AS saleQuantity,SUM(totalMoney) AS saleTotalMoney,d.contract_date
FROM buy_contract_item c
JOIN buy_contract d ON c.contract_id = d.id
WHERE 1=1 and (d.contract_date >= ‘2015-03-13‘ and d.contract_date <= ‘2015-03-15‘)
GROUP BY c.product_id;
) n ON t.id = n.productId
ORDER BY buyQuantity DESC, saleQuantity DESC
写的中间可能还有错误，但大致思路是这样，复杂的sql语句，可以先写整体框架，然后在框架里不断的细化查询。