[英]mySQL LEFT JOIN and COUNT number of occurrences in right hand table
以下mySQL查询返回一个行结构,该结构提供了票证表中的所有行以及销售表中的所有匹配信息。
SELECT tickets.*, sales.ID AS tcount
FROM tickets LEFT JOIN sales ON tickets.ID = sales.ticket_ID
但是,我们要做的就是为每张票计算右表中现有行数,以跟踪库存。
我以为这样做是可以的,但是尽管它正确地计算了票数,但它只从票证表中返回一行,并统计了tcount列中的所有销售额。
SELECT tickets.*,
COUNT(sales.ID) AS tcount
FROM tickets LEFT JOIN sales ON tickets.ID = sales.ticket_ID
您需要一个group by
:
SELECT tickets.*, COUNT(sales.ID) AS tcount
FROM tickets LEFT JOIN
sales
ON tickets.ID = sales.ticket_ID
GROUP BY tickets.ID;
切换表顺序或执行RIGHT JOIN
。 即:
SELECT tickets.*,
COUNT(sales.ID) AS tcount
FROM sales LEFT JOIN tickets ON tickets.ID = sales.ticket_ID
要么
SELECT tickets.*,
COUNT(sales.ID) AS tcount
FROM tickets RIGHT JOIN sales ON tickets.ID = sales.ticket_ID
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