COUNT(*) does not require an expression parameter because, by definition, it does not use information about any particular column. It counts the total number of rows in a table. This statement finds the total number of books in titles:
SELECT COUNT(*) |
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FROM titles |
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----------- |
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18 |
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(1 row(s) affected) |
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COUNT(*) returns the number of rows in the specified table without eliminating duplicates. It counts each row separately, including rows that contain null values.
Like other aggregate functions, COUNT(*) can be combined with other aggregate functions in the select list, with WHERE clauses, and so on. This statement shows COUNT(*) combined with a WHERE clause:
SELECT COUNT(*), AVG(price) |
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FROM titles |
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WHERE advance > $1000 |
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--------------- |
-------------- |
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15 |
24.20 |
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(1 row(s) affected) |
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