Unknown Column In Where Clause

SQL is evaluated backwards, from right to left. So the where clause is parsed and evaluate prior to the select clause. Because of this the aliasing of u_name to user_name has not yet occurred.

See the following MySQL manual page: http://dev.mysql.com/doc/refman/5.0/en/select.html

"A select_expr can be given an alias using AS alias_name. The alias is used as the expression's column name and can be used in GROUP BY, ORDER BY, or HAVING clauses."

(...)

It is not permissible to refer to a column alias in a WHERE clause, because the column value might not yet be determined when the WHERE clause is executed. See Section B.5.4.4, “Problems with Column Aliases”.

select u_name as user_name from users where u_name = "john";

Think of it like this, your where clause evaluates first, to determine which rows (or joined rows) need to be returned. Once the where clause is executed, the select clause runs for it.

To put it a better way, imagine this:

select distinct(u_name) as user_name from users where u_name = "john";

You can't reference the first half without the second. Where always gets evaluated first, then the select clause.

Either:

SELECT u_name AS user_name
FROM   users
WHERE  u_name = "john";

or:

SELECT user_name
from
(
SELECT u_name AS user_name
FROM   users
)
WHERE  u_name = "john";

The latter ought to be the same as the former if the RDBMS supports predicate pushing into the in-line view.

corrected:

SELECT u_name AS user_name FROM users WHERE u_name = 'john';

No you need to select it with correct name. If you gave the table you select from an alias you can use that though.

No you cannot. user_name is doesn't exist until return time.

SELECT user_name
FROM
(
SELECT name AS user_name
FROM   users
) AS test
WHERE  user_name = "john"

Unknown column in WHERE clause caused by lines 1 and 2 and resolved by line 3:

1. $sql = "SELECT * FROM users WHERE username =".$userName;
2. $sql = "SELECT * FROM users WHERE username =".$userName."";
3. $sql = "SELECT * FROM users WHERE username ='".$userName."'";

While you can alias your tables within your query (i.e., "SELECT u.username FROM users u;"), you have to use the actual names of the columns you're referencing. AS only impacts how the fields are returned.