MySQL stored function to read column from row in table - mysql

Question

We need a function / stored procedure to return a column in a row in a table, where we pass the table, id field name, id and column to return.
We have a table which is an activity log with columns such as
log_datetime - the date/time the user did something
log_table - the table affected
log_idcol - the column in log_table that holds the id
log_id - the id of the row in log_table
log_titlecol - the column in log_table which holds the 'title' of the
item
So in a report/ MySql statement I need to be able to list the title which is held in the column name held in log_titlecol, read from the table with the name in the log_table column, and id of whatever is in log_idcol.
so like
SELECT log_titlecol FROM log_table where log_idcol = log_id
where all the 'log_; parts are replaceable. I can't see a way to do this in MySQL, so is there a way to do this in a stored procedure/function?
For example:
CREATE TABLE jokes (
JokeID smallint(4) NOT NULL, **(autoincrement, unique)**
Title varchar(50) CHARACTER SET utf8 DEFAULT NULL,
Note text CHARACTER SET utf8
)
CREATE TABLE idioms (
IdiomID smallint(4) NOT NULL, **(autoincrement, unique)**
IdiomTitle varchar(50) CHARACTER SET utf8 DEFAULT NULL,
Note text CHARACTER SET utf8
)
CREATE TABLE `log` (
id smallint(4) NOT NULL, **(autoincrement, unique)**
log_datetime datetime,
log_table varchar(50) CHARACTER SET utf8 DEFAULT NULL,
log_idcol varchar(50) CHARACTER SET utf8 DEFAULT NULL,
log_id smallint(4) NOT NULL,
log_titlecol varchar(50) CHARACTER SET utf8 DEFAULT NULL,
)
INSERT INTO jokes (Title, Notes)
VALUES ('Funny joke','This is note1')
INSERT INTO jokes (Title, Notes)
VALUES ('Another Funny joke','This is another note')
INSERT INTO idioms (IdiomTitle, Notes)
VALUES ('Bird in the hand','What this means..')
INSERT INTO jokes (Title, Notes)
VALUES ('Another Funny joke','This is another note')
INSERT INTO log (log_datetime, log_table, log_idcol, log_id , log_titlecol)
VALUES (now(), 'jokes','JokeID',1,'Title')
INSERT INTO log (log_datetime, log_table, log_idcol, log_id , log_titlecol)
VALUES (now(), 'jokes','JokeID',2,'Title')
INSERT INTO log (log_datetime, log_table, log_idcol, log_id , log_titlecol)
VALUES (now(), 'idioms','IdiomID',2,'IdiomTitle')
So now how can I have a report from log showing the date time and the Title column from the Jokes table, row id as in the log_id column? I need a function.
eg SELECT log_datetime, log_table, log_id, GetTitle(log_table, log_idcol, log_id, log_titlecol) from log
Where GetTitle is a function which will return the column held in 'log_titlecol' form the table passed as log_table, from the row with an id (held in the log_idcol column) of log_id
So for example the output would show:
2018-01-01 12:00 jokes 1 Funny Joke
2018-01-01 12:10 jokes 2 Another Funny joke
2018-01-01 12:11 idioms 1 Bird in the hand
I have tried
CREATE PROCEDURE Getcol(IN tab TEXT CHARSET utf8mb4, IN col TEXT CHARSET utf8mb4, IN idcol TEXT CHARSET utf8mb4, IN id INT(15), OUT outcol TEXT CHARSET utf8mb4)
DETERMINISTIC
COMMENT 'Return a column from any table'
BEGIN
SET #Expression = CONCAT('SELECT ', col,' INTO #outcol FROM ', tab, ' where ', idcol, ' = ', id);
PREPARE myquery FROM #Expression;
EXECUTE myquery;
SELECT #outcol;
END
I can call this like
CALL GetCol('Jokes','Title','JokeID',1)
And this works to return the Title column for ID 1, but I am cant seem to then put this call into a function
CREATE FUNCTION getrowcol(tab TEXT, col TEXT, idcol TEXT, id INT) RETURNS text CHARSET utf8mb4
NO SQL
COMMENT 'Return a column from any table'
BEGIN
DECLARE outvar TEXT;
CALL GetCol(tab, col, idcol, id, #out1);
SELECT #out1 INTO outvar;
RETURN outvar;
END
This returns blank. How can I return #out1?
This is essentially the same question (with no answer)
https://dba.stackexchange.com/questions/151328/dynamic-sql-stored-procedure-called-by-a-function

Answer 1

Try something like,
DELIMITER //
CREATE PROCEDURE demo(tab VARCHAR(50), tit VARCHAR(50))
BEGIN
SET #Expression = CONCAT('SELECT l.log_datetime, j.',tit,' FROM `log` l INNER JOIN ',tab,' j;');
PREPARE myquery FROM #Expression;
EXECUTE myquery;
END
//
DELIMITER ;
Call it like
CALL demo('jokes', 'title')
from Reference