SQL:如何在SELECT查询结果中添加列? [英] SQL: How to add a column in the SELECT query result?

问题描述

通常，我们将在SQL查询中选择字段.例如

Usually we will select the field(s) in the SQL query. e.g.

SELECT A.id FROM Member A

但是，如果我要对齐元素与另一个所选字段相对应的列，该怎么办? 例如，我想从成员表中选择成员ID，然后选择COUNT，该计数计算成员在其他表的元组中出现的次数

But what if I want to align a column which elements correspond to the other selected field? For example I want to select the member ID from a member table, and the COUNT that count how many times the member appear in the tuple of other table

那么如何使COUNT列与选择结果对齐?

So how do I make the COUNT column that align together with the select result?

推荐答案

如果我对您的理解正确，这就是你想要的:

If I understood you correctly, this is what you want:

SELECT A.id, count(B.MemberID) 
FROM Member A 
LEFT JOIN TableB B on A.id = B.MemberID
group by A.id

LEFT JOIN将包括A中的记录，而B中没有任何对应的记录.而且，COUNT仅计算非空值，因此您需要将其与B.MemberID一起使用.这样，由于B.MemberID将是NULL，因此A中在B中没有任何对应记录的记录的计数将为0.

The LEFT JOIN will include records in A that do not have any corresponding records in B. Also, COUNT only counts non-null values, so you need to use it with B.MemberID. This way the count for records in A that do not have any corresponding records in B will be 0, since B.MemberID will be NULL.

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