need help on writing compound statements in mysql - mysql

Question

Is it possible to write compound statements in if condition -mysql ?
It is returning an error for me like:
[Err] 1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'DECLARE n INT unsigned DEFAULT 0;
DECLARE i INT unsigned DEFAULT 0;
DECL' at line 40
Sombody please help
Here is the query I am using
...........................................
IF(tarif='LT')
THEN
CREATE TABLE costSlabs SELECT `Start`,`End`,Cost FROM energy_slabs where SiteId=NEW.SiteID and `Start` < totalUnits;
DECLARE n INT DEFAULT 0;
DECLARE i INT DEFAULT 0;
DECLARE newStart INT;
DECLARE newEnd INT;
DECLARE newCost DOUBLE;
DECLARE finalCost DOUBLE DEFAULT 0;
SELECT COUNT(*) FROM costSlabs INTO n;
SET i=0;
WHILE i<n DO
SELECT `Start`, `End`,Cost INTO newStart,newEnd,newCost FROM costSlabs LIMIT i,1 ;
IF(newEnd<totalUnits)
THEN
SET finalCost = finalCost + ((newEnd-newStart) * newCost);
ELSE
...........................................

Answer 1

The IF statement is valid within the context of a MySQL stored program (stored procedure, function or trigger). Outside of the context of a MySQL stored program, it isn't a valid SQL statement.
We're not seeing the context of these statements, but based on the reference to NEW. in the SQL SELECT, without any table or table alias named NEW, I'll assume that this SQL is in the context of a CREATE TRIGGER statement.
I suspect that the problem you are encountering is due to the semicolon delimiter, which effectively ends the CREATE TRIGGER statement. In order to create a trigger that includes semicolon characters, it's necessary to specify a different statement delimiter. For example:
DELIMITER $$
CREATE TRIGGER mytrig
BEFORE INSERT ON mytable
FOR EACH ROW
BEGIN
SET NEW.col1 = 'foo';
SET NEW.col2 = 'bar';
END$$
DELIMITER ;
The first statement specifies that $$ will be used as the delimiter to end a statement. With that set, the statements following will be read together as a single statement, until a $$ delimiter is encountered. That will end the statement, and allow it be executed. This basically prevents the semicolons within the trigger body from prematurely "ending" the statement.
Once the statements are completed, you can change the delimiter back to a semicolon.
The block of SQL text posted in the question is invalid on its own. I suspect that the syntax error is being returned from the execution of a CREATE TRIGGER statement, but that's just a guess, since there's not enough context in the question to make a conclusion.
Also, I believe that within a MySQL stored program (such as a TRIGGER), all of the DECLARE statements must appear before other statements, such as CREATE TABLE.