SQL 使用 subselect 来完成 LEFT JOIN
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Using subselect to accomplish LEFT JOIN
提问by Andre
Is is possible to accomplish the equivalent of a LEFT JOIN with subselect where multiple columns are required. Here's what I mean.
在需要多列的情况下，是否可以使用子选择完成等效的 LEFT JOIN。这就是我的意思。
SELECT m.*, (SELECT * FROM model WHERE id = m.id LIMIT 1) AS models FROM make m
As it stands now doing this gives me a 'Operand should contain 1 column(s)' error.
就目前而言，这样做会给我一个“操作数应该包含 1 列”错误。
Yes I know this is possible with LEFT JOIN, but I was told it was possible with subselect to I'm curious as to how it's done.
是的，我知道使用 LEFT JOIN 可以做到这一点，但有人告诉我可以使用 subselect，因为我很好奇它是如何完成的。
采纳答案by MisterZimbu
A subselect can only have one column returned from it, so you would need one subselect for each column that you would want returned from the model table.
子选择只能从中返回一列，因此您需要为要从模型表中返回的每一列进行一个子选择。
回答by mechanical_meat
There are many practical uses for what you suggest.
你的建议有很多实际用途。
This hypothetical query would return the most recent release_date(contrived example) for any make with at least one release_date, and null for any make with no release_date:
这个假设的查询将为release_date任何至少有一个的 make返回最新的（人为的例子），对于任何没有的 make返回release_datenull release_date：
SELECT m.make_name,
sub.max_release_date
FROM make m
LEFT JOIN
(SELECT id,
max(release_date) as max_release_date
FROM make
GROUP BY 1) sub
ON sub.id = m.id
