如何遍历两个表中的行并基于SQL中的合并结果创建新集

Question

Here is my obstacle.

I have two tables. Table A contains more rows than Table B. I have to merge the results and if Table A does not contain a row from Table B then I insert it into the new set. If however, a row from Table A contains a row with the same primary key as Table B, the new set will take the row from Table B.

Would this best be done in a cursor or is there an easier way to do this? I ask because there are 20 million rows and while I am new to sql, i've heard cursors are expensive.

Answer 1

Your phrasing is a little vague. It seems that you want everything from TableB and then rows from TableA that have no matching primary key in B. The following query solves this problem:

select *
from tableB union all
select *
from tableA
where tableA.pk not in (select pk from tableB)

Answer 2

Yep, cursors are expensive.

There's a MERGE command in later versions of SQL that will do this in one shot, but it's sooo cumbersome. Better to do it in two pieces - first:

UPDATE A SET
    field1 = B.field1
   ,field2 = B.field2
   , etc
FROM A JOIN B on B.id = A.id

Then:

INSERT A SELECT * FROM B --enumerate fields if different
WHERE B.id not in (select id FROM A)

Answer 3

An OUTER JOIN should do what you need and be more efficient than a cursor. Try this query

--first get the rows that match between TableA and TableB
INSERT INTO [new set]
SELECT TableB.* --or columns of your choice
FROM TableA LEFT JOIN TableB ON [matching key criteria]
WHERE TableB.[joining column/PK] IS NOT NULL

--then get the rows from TableA that don't have a match
INSERT INTO [new set]
SELECT TableA.* --you didn't say what was inserted if there was no matching row 
FROM TableA LEFT JOIN TableB ON [matching key criteria]
WHERE TableB.[joining column/PK] IS NULL