Referring to a Column Alias in a WHERE Clause

sql

prompt-engineering

best-practices

performance

byAnton Shumikhin·Oct 8, 2024

Aliasing in SQL occurs late in the game; it's not present during the WHERE clause. Apply subqueries or replicate expressions, like so:

-- Subquery encapsulates alias
SELECT * FROM (
  SELECT col1, col2, col1 + col2 AS total 
  FROM your_table
) AS sub 
WHERE sub.total > 100;

-- Expression calculates directly 
SELECT col1, col2, col1 + col2 AS total 
FROM your_table 
WHERE col1 + col2 > 100;

Deep Dive: Alternatives and Workarounds

Usually, the duplication of expressions or subqueries suffice. For more intricate scenarios, let's explore advanced methods.

CTEs: Modular and Readable

A Common Table Expression (CTE) is like creating a mini, disposable view that survives only the lifetime of the query. And yes, you can use aliases here:

WITH CTE AS (
    SELECT col1, col2, DATEDIFF(day, col2, GETDATE()) AS daysdiff 
    FROM your_table
)
SELECT * FROM CTE 
WHERE daysdiff > 100;

HAVING: The LifeSaver Post-Aggregation

HAVING clause to the rescue when you want to reference aliases post-aggregation:

SELECT col1, DATEDIFF(day, maxlogtm, GETDATE()) AS daysdiff
FROM your_table
GROUP BY col1
HAVING daysdiff > 120;

Time Travel with DATEDIFF

Dating is complicated, and so is calculating day difference. Our friend DATEDIFF function can help, especially when dealing with the current date using GETDATE():

-- Just another date in the WHERE clause
SELECT col1, DATEDIFF(day, maxlogtm, GETDATE()) AS daysdiff
FROM your_table
WHERE DATEDIFF(day, maxlogtm, GETDATE()) > 120;

Familiarize yourself with DATEDIFF and its proper syntax to avoid awkward date computations.

Juggling Complex Logical Conditions

Some queries feel like solving a Rubik's cube. For such challenging logic involving aliases, use outer apply or subquery as a derived table:

-- OUTER APPLY: WHEN SUBQUERY is too mainstream
SELECT main.*, ext.daysdiff
FROM your_table main
OUTER APPLY (SELECT DATEDIFF(day, main.maxlogtm, GETDATE()) AS daysdiff) ext
WHERE ext.daysdiff > 100;

And don't ever forget to invite parenthesis to the party for organizing the logical conditions.

Performance: Aliasing and Efficiency

HAVING can sometimes pull off a better performance than a WHERE clause while dealing with column aliases in aggregated data. This might also save you from repeating the code.

Choose the Right Tool (or SQL Function)

Ensure compatibility of SQL Functions with your database system. Because not all environments welcome all functions.

Outsmarting `WHERE` Clause Limitations

When the WHERE clause plays coy, move towards more advanced, versatile solutions.

Jazzing up with `GROUP BY` and `HAVING`

Pivot to HAVING when filtering by an aggregate alias.

-- HAVING: Because WHERE   was not enough.
SELECT col1, SUM(col2) AS total
FROM your_table
GROUP BY col1
HAVING total > 100;

The Unexpected Hero: `OUTER APPLY`

This bridge between SELECT and WHERE clauses allows you to filter by alias directly, like a WHERE with privileges:

SELECT main.*, sub.finding
FROM your_table main
OUTER APPLY (SELECT sum(main.col2) AS finding) sub
WHERE sub.finding > 100;

Condition your Conditions: The Clause Order

The sequence of SQL clauses is crucial for understanding the limitations of WHERE while handling aliases:

FROM
WHERE
GROUP BY
HAVING
SELECT

The SELECT clause (where we define our alias) happens after the WHERE clause, causing the alias to be unrecognized during the WHERE filtering.

explain-codes / Sql / Referring to a Column Alias in a WHERE Clause

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Deep Dive: Alternatives and Workarounds Time Travel with DATEDIFF Juggling Complex Logical Conditions Performance: Aliasing and Efficiency Choose the Right Tool (or SQL Function)Outsmarting Clause Limitations Condition your Conditions: The Clause Order