连接
功能介绍：
将多个表按照维进行对齐关联，这种对齐方式是用户不需要关心表之间的连接关系，可以为每张表独立设定汇总维度。
语法描述：
SELECT D.F, T1.f(...), T2.f(...)
ON D AS A
FROM T1 WHERE… BY ...
JOIN / LEFT JOIN / FULL JOIN T2 WHERE ... BY......
JOIN / LEFT JOIN / FULL JOIN Tn
或
UNION T2 WHERE ... BY......
举例：
|
SELECT ReturnedPmt.sum(Amount) Pamount, Orders.sum(Amount) Oamount ON YearMonth FROM Orders BY ShipDate#YearMonth UNION ReturnedPmt BY RDate#YearMonth |
多表关联，按月份统计订单总额和回款总额。 |
|
SELECT ReturnedPmt.sum(Amount) Pamount, Orders.sum(Amount) Oamount ON year,EMPLOYEE FROM Orders BY ShipDate#year,EmployeeID UNION ReturnedPmt BY RDate#year ,SellerID |
多字段关联，按照年份和雇员ID统计订单总额和回款总额。 |
|
SELECT ReturnedPmt.sum(Amount) Pamount, Orders.sum(Amount) Oamount ON YearMonth FROM ReturnedPmt BY RDate#YearMonth Left Join Orders BY ShipDate#YearMonth |
左连接。 |
知识点：
1）DQL中的UNION相当于SQL中的FULL JOIN连接。
2）在多表关联的时候，需要写明字段属于哪张表，并且需要在BY后面写齐字段。