MySQL local variables

问题:

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?

回答1:

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;

回答2:

Try this:-

 select @countTotal := COUNT(*) from nGrams;

回答3:

Function example:

DROP FUNCTION IF EXISTS test;

DELIMITER $$
CREATE FUNCTION test(in_number INT) RETURNS INT
    BEGIN
        DECLARE countTotal INT;
        SET countTotal = SELECT COUNT(*) FROM nGrams;
    RETURN countTotal + in_number;
END $$
DELIMITER ;

回答4:

According to DECLARE Syntax, declare must be inside a begin...end block.