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Rechercher dans le manuel MySQL
B.4.4.4 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:
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:
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:
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`
:
But this statement groups by the literal string
'a'
and will not work as expected:
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Steph