Why Does MySQL Throw an 'Unknown Column' Error When Using CONCAT in a WHERE Clause?

MySQL: Troubleshooting the "Unknown Column" Error in CONCAT Condition

In MySQL, the CONCAT function allows you to concatenate multiple values into a single expression. However, users may encounter the "unknown column" error when trying to use the function as a condition in a WHERE clause.

Understanding the Issue

The error occurs because the alias assigned to the concatenated value (e.g., firstlast) is not recognized as a valid column within the query. This is due to the fact that aliases are only applied to the output of a query and are not available within the query itself.

Resolution

To resolve this issue, there are two possible approaches:

1. Repeating the Expression:

One way to avoid the error is to repeat the CONCAT expression in the WHERE clause:

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

This method ensures that the concatenated value is compared correctly against the desired input.

2. Wrapping the Query:

Alternatively, you can wrap the original query in a subquery and create an alias for the concatenated value within the inner query:

SELECT * FROM (
  SELECT neededfield, CONCAT(firstname, ' ', lastname) AS firstlast 
  FROM users
) AS base 
WHERE firstlast = "Bob Michael Jones";

In this case, the alias firstlast is assigned to the concatenated value within the subquery, allowing it to be used as a condition in the outer query.

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