Question

I use a cursor to iterate through quite a big table. For each row I check if value from one column exists in other.

If the value exists, I would like to increase value column in that other table.
If not, I would like to insert there new row with value set to 1.

I check “if exists” by:

IF (SELECT COUNT(*) FROM otherTabe WHERE... > 1)
   BEGIN
      ...
   END
ELSE
   BEGIN
      ...
   END

I don’t know how to get that row which was found and update value. I don’t want to make another select.

How can I do this efficiently?

I assume that the method of checking described above isn’t good for this case.

Answer 1

Depending on the size of your data and the actual condition, you have two basic approaches:

1) use MERGE

MERGE TOP (...) INTO table1
USING table2 ON table1.column = table2.column
WHEN MATCHED
 THEN UPDATE SET table1.counter += 1
WHEN NOT MATCHED SOURCE
 THEN INSERT (...) VALUES (...);

the TOP is needed because when you’re doing a huge update like this (you mention the table is ‘big’, big is relative, but lets assume truly big, +100MM rows) you have to batch the updates, otherwise you’ll overwhelm the transaction log with one single gigantic transaction.

2) use a cursor, as you are trying. Your original question can be easily solved, simply always update and then check the count of rows updated:

UPDATE table 
  SET column += 1
WHERE ...;
IF @@ROW_COUNT = 0
BEGIN
   -- no match, insert new value 
   INSERT INTO (...) VALUES (...);
END

Note that this approach is dangerous though because of race conditions: there is nothing to prevent another thread from inserting the value concurrently, so you may end up with either duplicates or a constraint violation error (preferably the latter…).