CodeKingPlusPlus CodeKingPlusPlus - 7 months ago 21
SQL Question

MySQL local variables

I am trying to define and initialize a MySQL variable for a query.

I have the following:

declare @countTotal int;
SET @countTotal = select COUNT(*)
from nGrams;


I am using MySQL in Netbeans and it tells me I have an error. What/where is my error?

How can I fix this?

Answer

MySQL has two different types of variable:

  • local variables (which are not prefixed by @) are strongly typed and scoped to the stored program block in which they are declared. Note that, as documented under DECLARE Syntax:

    DECLARE is permitted only inside a BEGIN ... END compound statement and must be at its start, before any other statements.

  • user variables (which are prefixed by @) are loosely typed and scoped to the session. Note that they neither need nor can be declared—just use them directly.

Therefore, if you are defining a stored program and actually do want a "local variable", per the wording in your question, you will need to drop the @ character and ensure that your DECLARE statement is at the start of your program block. Otherwise, to use a "user variable", drop the DECLARE statement.

Furthermore, you will either need to surround your query in parentheses in order to execute it as a subquery:

SET @countTotal = (SELECT COUNT(*) FROM nGrams);

Or else, you could use SELECT ... INTO (but only with a user variable):

SELECT COUNT(*) INTO @countTotal FROM nGrams;