如何用表B中的数据更新表A？

Question

I am trying to update table A with two data values from table B. I thought I could do something like:

UPDATE A
SET A.DATA1= B.DATA1, A.DATA2= B.DATA2
FROM TABLE1 A, TABLE2 B
WHERE A.ID = B.ID;

What is the correct syntax?

Answer 1

The classic (non-obvious) standard SQL answer is:

UPDATE A
   SET (Data1, Data2) = ((SELECT B.Data1, B.Data2
                            FROM B
                           WHERE B.ID = A.ID))
 WHERE A.ID IN (SELECT B.ID FROM B);

The correlated sub-query generates one result row (presumably) from B for each matching row in A. The WHERE clause on the UPDATE prevents you from nullifying data A where there isn't a matching row in B. The double parentheses aren't a casual typo.

This is messy to write, so most DBMS provide a non-standard mechanism to do updates with a join, but the syntax used varies by DBMS.

Answer 2

For SQL Server I would do:

UPDATE A SET
    A.DATA1= B.DATA1,
    A.DATA2= B.DATA2
FROM TABLE1 A
JOIN TABLE2 B on WHERE A.ID = B.ID