How to Concatenate and Compare Strings Using MySQL SELECT with CONCAT: Avoiding the 'Unknown Column” Error?

Using MySQL SELECT with CONCAT Condition

When working with a table containing first and last name fields, a common task is to concatenate them into a single string for comparison or filtering purposes. However, encountering an "unknown column" error while using an alias for the concatenated string can be puzzling.

To overcome this, it's essential to understand that aliases are only applicable to the output of a query, not within the query itself. To concatenate and compare strings, you have two options:

Repeating the Concatenation:

SELECT neededfield, CONCAT(firstname, ' ', lastname) as firstlast
FROM users
WHERE CONCAT(firstname, ' ', lastname) = "Bob Michael Jones"

By repeating the concatenation expression in the WHERE clause, you ensure that you're comparing the actual concatenated values.

Using a Subquery:

SELECT * FROM (
    SELECT neededfield, CONCAT(firstname, ' ', lastname) as firstLast
    FROM users
) base
WHERE firstLast = "Bob Michael Jones"

By wrapping the query in a subquery, you create a temporary table with an alias ("base" in this example) that includes the concatenated string as a column. You can then filter this temporary table using the desired comparison.

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