Explain the function of each of the clauses in the SELECT statement. What restrictions are imposed on these clauses?
FROM Specifies the table or tables to be used.
WHERE Filters the rows subject to some condition.
GROUP BY Forms groups of rows with the same column value.
HAVING Filters the groups subject to some condition.
SELECT Specifies which columns are to appear in the output.
ORDER BY Specifies the order of the output.
If the SELECT list includes an aggregate function and no GROUP BY clause is being used to
group data together, then no item in the SELECT list can include any reference to a column unless
that column is the argument to an aggregate function.
When GROUP BY is used, each item in the SELECT list must be single-valued per group.
Further, the SELECT clause may only contain:
? Column names.
? Aggregate functions.
? Constants.
? An expression involving combinations of the above.
All column names in the SELECT list must appear in the GROUP BY clause unless the name is
used only in an aggregate function.
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