Question

I understand that prepared statements aren’t allowed within a trigger, but I can’t see how else I can do what I want, so any alternatives/workarounds would be greatly appreciated.

DELIMITER $$

CREATE TRIGGER trigger1 after INSERT on table1
FOR EACH ROW
begin
DECLARE a varchar(30);
DECLARE b varchar(30);
DECLARE c varchar(30);
DECLARE d varchar(30);
SET a = new.Col1;
SET b = new.Col2;
SET c = (select Col4 from table2 where Col3=new.Col3);
SET d = concat('select ',a,' from table3 where yq=',c,' and xz=',b);
insert into table4 values (d);
end
$$

DELIMITER;

So what is happening: a row is inserted and 3 values are taken from that row, Col1,Col2, and new.Col3.
new.Col3 is used to get a row from a different table, which is then used along with Col2 to search the 3rd table where the value I need is.
Col1 is the problem because it is supposed to be a dynamically generated column name for d but this doesn’t work.
So obviously at the moment all I am getting back is a string of the select statement in d which is what I expect, but I cant execute the statement because that is not allowed.
So how can I get that dynamically generated column name in to the select statement?

Table 1

Col1 | Col2 | Col3
c    | d    | x
a    | e    | y
b    | e    | z
b    | f    | x

Table 2

Col3 | Col4
x    | 1
y    | 2 
z    | 3

Table3

a    | b    | c    | xz    | yq
1a   | 1b   | 1c   |  d    | 1
2a   | 2b   | 2c   |  d    | 2
3a   | 3b   | 3c   |  d    | 3
4a   | 4b   | 4c   |  e    | 1
5a   | 5b   | 5c   |  e    | 2
6a   | 6b   | 6c   |  e    | 3
7a   | 7b   | 7c   |  f    | 1
8a   | 8b   | 8c   |  f    | 2
9a   | 9b   | 9c   |  f    | 3

The result I want is from 1a-9c.

I have considered changing the table schema so that the column name in the trigger can be fixed, but that would be a last resort since it would cause issues for other bits of code.

Answer 1

You will have to abandon the idea of prepared statements and dynamic SQL altogether and just painfully do some IF..THEN statements:

DELIMITER $$

CREATE TRIGGER trigger1 after INSERT on table1
FOR EACH ROW
begin
DECLARE a varchar(30);
DECLARE b varchar(30);
DECLARE local_c varchar(30);
DECLARE d varchar(30);
SET a = new.Col1;
SET b = new.Col2;
SET local_c = (select Col4 from table2 where Col3=new.Col3);
IF local_c = 'a' THEN
    select a into d from table3 where yq=local_c and xz=b;
END IF;
IF local_c = 'b' THEN
    select b into d from table3 where yq=local_c and xz=b;
END IF;
IF local_c = 'c' THEN
    select c into d from table3 where yq=local_c and xz=b;
END IF;
IF ISNULL(d) = 0 THEN
    insert into table4 values (d);
END IF;
end
$$

DELIMITER;