Referencing an Aliased Column in the WHERE Clause -SQL

Problem

You have used aliases to provide more meaningful column names for your result set and would like to exclude some of the rows using the WHERE clause. However, your attempt to reference alias names in the WHERE clause fails:

 select sal as salary, comm as commission
   from emp
  where salary <>

Solution

By wrapping your query as an inline view you can reference the aliased columns:

 1 select *
 2   from (
 3 select sal as salary, comm as commission
 4   from emp
 5        ) x
 6  where salary <>

iscussion

In this simple example, you can avoid the inline view and reference COMM or SAL directly in the WHERE clause to achieve the same result. This solution introduces you to what you would need to do when attempting to reference any of the following in a WHERE clause:

• Aggregate functions
• Scalar subqueries
• Windowing functions
• Aliases

Placing your query, the one giving aliases, in an inline view gives you the ability to reference the aliased columns in your outer query. Why do you need to do this? The WHERE clause is evaluated before the SELECT, thus, SALARY and COMMISSION do not yet exist when the "Problem" query's WHERE clause is evaluated. Those aliases are not applied until after the WHERE clause processing is complete. However, the FROM clause is evaluated before the WHERE. By placing the original query in a FROM clause, the results from that query are generated before the outermost WHERE clause, and your outermost WHERE clause "sees" the alias names. This technique is particularly useful when the columns in a table are not named particularly well.

TIPS :
The inline view in this solution is aliased X. Not all databases require an inline view to be explicitly aliased, but some do. All of them accept it.
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