MySQL local variables

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失恋的感觉
失恋的感觉 2021-02-05 02:44

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         


        
4条回答
  •  长发绾君心
    2021-02-05 03:34

    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:

    SELECT COUNT(*) INTO @countTotal FROM nGrams;
    

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