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mysqlmergecoalesceifnull

MySQL: Merge rows and returning default values

Imagine the following table:

id | variant | name
-----------------------
1  | 1       | a
1  | 2       | b
1  | 3       | c
2  | 1       | d
2  | 2       | e
2  | 3       | NULL
3  | 1       | g

Which SQL statement do I need to run to get this:

  • For a given id and a given variant get the name of this combination.
  • But if the name is NULL get the name of the row with the given id but with variant 1 (the name of variant 1 is never NULL).
  • If there is no row with the given variant, again, use the row where variant is 1 with the same id.
  • Variant 1 is never requested directly.

This is like a fall back mechanism. Or you could consider it as overriding values of rows with variant = 1.

Examples:

  • id = 1, variant = 2: b
  • id = 1, variant = 3: c
  • id = 2, variant = 3: d
  • id = 3, variant = 5: g

Is this possible with SQL? And is it performing well if the mechanism is applied on many more fields?

Thanks!

Update:

Please note that I would like to have this mechanism not only for the name field. There should be further columns which should have the same behaviour - but each column should be treated on its own.

Solution

  • This should do what you need using a LEFT JOIN to get the optional value.

    SELECT COALESCE(b.Name,a.Name)
FROM Table1 a
LEFT JOIN Table1 b
  ON a.id=b.id AND b.variant=@y
WHERE a.id=@x AND a.variant=1

    An SQLFiddle to test with.

    Performance wise, it would depend on how you need to apply the query to get multiple fields. If you can solve your column choice using COALESCE from the existing join, I can't see a big problem, but if you end up with multiple self joins to solve it, you may have a problem.