As described in Problems with Column Aliases:
An alias can be used in a query select list to give a column a different name. You can use the alias in GROUP BY
, ORDER BY
, or HAVING
clauses to refer to the column:
SELECT SQRT(a*b) AS root FROM tbl_name
GROUP BY root HAVING root > 0;
SELECT id, COUNT(*) AS cnt FROM tbl_name
GROUP BY id HAVING cnt > 0;
SELECT id AS 'Customer identity' FROM tbl_name;
Standard SQL disallows references to column aliases in a WHERE
clause. This restriction is imposed because when the WHERE
clause is evaluated, the column value may not yet have been determined. For example, the following query is illegal:
SELECT id, COUNT(*) AS cnt FROM tbl_name
WHERE cnt > 0 GROUP BY id;
The WHERE
clause determines which rows should be included in the GROUP BY
clause, but it refers to the alias of a column value that is not known until after the rows have been selected, and grouped by the GROUP BY
.