How do I use alias in where clause?

sql

subqueries

performance-optimization

sql-server

byNikita Barsukov·Jan 6, 2025

To utilize an alias in an SQL filter, you have to refer to the primary expression in the WHERE clause or implement a subquery or common table expression (CTE). Implementing aliases in the WHERE clause is not permissible because the SELECT clause is processed after WHERE.

Below are examples for better understanding: Subquery Example:

/* 
when the sun of "total_sum" shines brighter 
than 100 lumen
*/
SELECT sub.*
FROM (
    SELECT col, SUM(col2) AS total_sum
    FROM table
) AS sub
WHERE sub.total_sum > 100;

CTE Example:

/* 
when total_sum decides to take the road less 
traveled by and it's > 100
*/
WITH TotalSums AS (
    SELECT col, SUM(col2) AS total_sum
    FROM table
)
SELECT *
FROM TotalSums
WHERE total_sum > 100;

Bear in mind that while SQL Server prohibits the usage of column aliases in the WHERE clause, MySQL avails some latitude, letting aliases be recycled in GROUP BY, ORDER BY, and HAVING clauses.

Understanding SQL Phases

To sufficiently comprehend why aliases may not be employed in the WHERE clause, one should understand SQL's processing order. It typically follows this sequence:

FROM
WHERE
GROUP BY
HAVING
SELECT
ORDER BY

Aliases are defined in the SELECT phase which comes after the WHERE clause, thus you can't use a select-level alias within the WHERE clause.

Efficient Performance with Subqueries

Concerning performance optimization, subqueries play a significant role. You may enhance execution times by incorporating solely the necessary columns in a sub-select, then filtering in an outer query. It's also worth noting that sophisticated subqueries could sometimes degrade performance compared to alternative solutions — keep testing.

Crafty Use of Cross Apply

In some DBMS like SQL Server, the CROSS APPLY operator is handy for referencing an alias in the WHERE clause. The operator functions like a join, enabling the generation of column alias which can be referenced in the query.

Optimizing Query Efficiency

During your query formulation, the sequence of conditions hugely influences performance. SQL Server and others might utilize short-circuit logic, assessing more selective or affordable conditions early on to trim down the processed rows.

Circumventing WITH HAVING

In MySQL, if dealing with aggregate functions you can surpass the WHERE clause alias limitation with the HAVING clause. It filters aggregated data and is able to reference select-level aliases, since it is processed post the SELECT phase.

Venturing into Documentation

Diving into the DBMS-specific documentation is vital when handling aliases, especially in intricate scenarios. As each database system handles aliasing and filtering uniquely, acquainting yourself prevents daunting debugging hours.

explain-codes / Sql / How do I use alias in where clause?

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