Why Does MySQL Throw an 'Unknown Column' Error When Using CONCAT in SELECT Queries?

Understanding CONCAT Conditions in MySQL SELECT Queries

When dealing with fields representing names or other concatenated information, MySQL's CONCAT function provides a versatile tool for combining values and performing comparisons. However, using CONCAT in SELECT queries can lead to the error "unknown column" for the alias assigned to the concatenated expression.

To overcome this error, it's important to understand that aliases in SELECT queries are only applicable to the output columns and are not recognized within the query itself. Therefore, to use a CONCAT expression in a WHERE condition, you have two options:

1. Repeat the CONCAT Expression:

   Instead of using the alias, repeat the CONCAT expression within the WHERE clause:

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

2. Wrap the Query:

   Alternatively, you can wrap the original query into a subquery and assign an alias to the CONCAT expression in the subquery:

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

In both cases, the query successfully compares the concatenated value of the firstname and lastname fields with the provided string, "Bob Michael Jones," and retrieves the desired results.

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