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Message Posted: Thu, 09 Jun 2011 @ 10:20:03 GMT

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Count(*) counts all occurrences including nulls. This is a row count.

- Count (x) counts all non null occurrences of values in column x. If x has null values, count(x) will be less than count(*).

- Count (1) is like count(x) it will count all null values in a "pseudo column" created by the constant. Since the constant 1 will never be null, this would, like count(*), give you a row count.

Thus with this data in t3:

     X       y
     1       1
     Null    2
     Null    3
     4       4

And this query:

     select count(*), count(x), count(1)
     from t3;

you get:

     4, 2, 4

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