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B.4.4.4 Problems with Column Aliases

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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:

  1. SELECT SQRT(a*b) AS root FROM tbl_name
  2.   GROUP BY root HAVING root > 0;
  3. SELECT id, COUNT(*) AS cnt FROM tbl_name
  4.   GROUP BY id HAVING cnt > 0;
  5. 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:

  1. SELECT id, COUNT(*) AS cnt FROM tbl_name
  2.   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.

In the select list of a query, a quoted column alias can be specified using identifier or string quoting characters:

  1. SELECT 1 AS `one`, 2 AS 'two';

Elsewhere in the statement, quoted references to the alias must use identifier quoting or the reference is treated as a string literal. For example, this statement groups by the values in column id, referenced using the alias `a`:

  1. SELECT id AS 'a', COUNT(*) AS cnt FROM tbl_name
  2.   GROUP BY `a`;

But this statement groups by the literal string 'a' and will not work as expected:

  1. SELECT id AS 'a', COUNT(*) AS cnt FROM tbl_name
  2.   GROUP BY 'a';

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Document created the 26/06/2006, last modified the 26/10/2018
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