Question

我有两张桌子，一张包含顾客，另一张包含预订现在我想知道有多少预订来自一个人，但是用他们的名字而不是id来显示。

SELECT booking.id, COUNT(booking.id) AS idcount
FROM booking
GROUP BY booking.id ORDER BY idcount DESC;

输出是（正确计数）：

       id | idcount 
----------+--------
        2 |      8
        1 |      4

我尝试显示名称而不是ID：

SELECT customer.lastn, customer.firstn, COUNT(booking.id) AS idcount
FROM booking, customer
GROUP BY customer.lastn, customer.firstn ORDER BY idcount DESC;

输出（错误计数）：

 lastn    | firstn  | idcount 
----------+---------+--------
 Adam     | Michael |     13
 Jackson  | Leo     |     13

13是预订总数（我只是关闭了输出）所以有来自的，但是我无法通过名称进行转换以获得正确的计数。

Answer 1

您需要在FROM子句中使用JOIN：

SELECT customer.lastn, customer.firstn, COUNT(booking.id) AS idcount
FROM booking
    JOIN customer ON booking.id = customer.id
GROUP BY customer.lastn, customer.firstn 
ORDER BY idcount DESC;

此处的JOIN说明了booking表与customer表的关系。

从postgres中选择Query with count

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