如何在不使用JOIN的情况下实现SQL连接?

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不使用JOIN关键字如何实现SQL连接?

How does one implement SQL joins without using the JOIN keyword?

这并不是真的必要，但是我认为通过这样做，我可以更好地理解联接的实际作用.

This is not really necessary, but I thought that by doing this I could better understand what joins actually do.

推荐答案

基本的INNER JOIN易于实现. 以下:

The basic INNER JOIN is easy to implement. The following:

SELECT L.XCol, R.YCol FROM LeftTable AS L INNER JOIN RightTable AS R ON L.IDCol=R.IDCol;

等效于:

SELECT L.XCol, R.YCol FROM LeftTable AS L, RightTable AS R WHERE L.IDCol=R.IDCol;

要将其扩展到LEFT/RIGHT/FULL OUTER JOIN，您只需要将不匹配的行以及正确列中的NULL联合到先前的INNER JOIN.

In order to extend this to a LEFT/RIGHT/FULL OUTER JOIN, you only need to UNION the rows with no match, along with NULL in the correct columns, to the previous INNER JOIN.

对于左外部联接，添加:

For a LEFT OUTER JOIN, add:

UNION ALL SELECT L.XCol, NULL /* cast the NULL as needed */ FROM LeftTable AS L WHERE NOT EXISTS ( SELECT * FROM RightTable AS R WHERE L.IDCol=R.IDCol)

要进行正确的外部联接，请添加:

For a RIGHT OUTER JOIN, add:

UNION ALL SELECT NULL, R.YCol /* cast the NULL as needed */ FROM RightTable AS R WHERE NOT EXISTS ( SELECT * FROM LeftTable AS L WHERE L.IDCol=R.IDCol)

要进行外部联接，请同时添加以上两项.

For a FULL OUTER JOIN, add both of the above.