#1222 - The used SELECT statements have a different number of columns

Question

Why am i getting a #1222 - The used SELECT statements have a different number of columns ? i am trying to load wall posts from this users friends and his self.

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Answer 1

Beside from the answer given by @omg-ponies; I just want to add that this error also occur in variable assignment. In my case I used an insert; associated with that insert was a trigger. I mistakenly assign different number of fields to different number of variables. Below is my case details.

INSERT INTO tab1 (event, eventTypeID, fromDate, toDate, remarks)
    -> SELECT event, eventTypeID, 
    -> fromDate, toDate, remarks FROM rrp group by trainingCode;
ERROR 1222 (21000): The used SELECT statements have a different number of columns

So you see I got this error by issuing an insert statement instead of union statement. My case difference were

1. I issued a bulk insert sql

   i.e. insert into tab1 (field, ...) as select field, ... from tab2

2. tab2 had an on insert trigger; this trigger basically decline duplicates

It turns out that I had an error in the trigger. I fetch record based on new input data and assigned them in incorrect number of variables.

DELIMITER @@
DROP TRIGGER trgInsertTrigger @@
CREATE TRIGGER trgInsertTrigger
BEFORE INSERT ON training
FOR EACH ROW
BEGIN
SET @recs = 0;
SET @trgID = 0;
SET @trgDescID = 0;
SET @trgDesc = '';
SET @district = '';
SET @msg = '';

SELECT COUNT(*), t.trainingID, td.trgDescID, td.trgDescName, t.trgDistrictID
    INTO @recs, @trgID, @trgDescID, @proj, @trgDesc, @district
    from training as t
    left join trainingDistrict as tdist on t.trainingID = tdist.trainingID
    left join trgDesc as td on t.trgDescID = td.trgDescID
    WHERE
    t.trgDescID = NEW.trgDescID
    AND t.venue = NEW.venue
    AND t.fromDate = NEW.fromDate 
    AND t.toDate = NEW.toDate 
    AND t.gender = NEW.gender
    AND t.totalParticipants = NEW.totalParticipants
    AND t.districtIDs = NEW.districtIDs;

IF @recs > 0 THEN
    SET @msg = CONCAT('Error: Duplicate Training: previous ID ', CAST(@trgID AS CHAR CHARACTER SET utf8) COLLATE utf8_bin);
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = @msg;
END IF;
END @@ 

DELIMITER ; 

As you can see i am fetching 5 fields but assigning them in 6 var. (My fault totally I forgot to delete the variable after editing.