MYSQL concat char to create table name with select - mysql

Question

I tried to use such code
select
price
from
(select concat("_",`id`) -- to set table name
as
dta
from
druk
where
date >= '2021-02-01' and date < '2021-03-01') d
If I put * instead price I get for example "_5438" - table name. One or more. In this way I can't get price.
I tried to use variables from examples I found, but some of them mysql do'es not recognize. What should I do to make proper, dynamic table name with concat?

Answer 1

You can use dynamic sql for this.
First you get the table nane into #dta and after that constructs the actual query which you run as prepare statements
But this onlöy works, if your forwst select only goves back 1 eow as result, that is why it is LIMIT 1
SELECT concat("_",`id`)
INTO
#dta
FROM
druk
WHERE
date >= '2021-02-01' AND `date` < '2021-03-01' LIMIT 1;
SET #sql := CONCAT('SELECT price FROM ',#dta );
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
Still this does look like a poor design. to have many tables with the same design
For multiple ids you need a stored procedure, it can look like this).
But i didn_'t test it
CREATE TABLE druk(`id` INT , `date` date)
INSERT INTO druk VALUES (1,now()),(2,now())
CREATE TABLE _1 (price DECIMAL(8,3))
INSERT INTO _1 VALUE (2.3),(4.6),(7.9)
CREATE TABLE _2 (price DECIMAL(8,3))
INSERT INTO _2 VALUE (0.3),(1.6),(3.9)
CREATE PROCEDURE createpricelist( IN _datefrom varchar(29),IN _dateto varchar(20)
)
BEGIN
DECLARE finished INTEGER DEFAULT 0;
DECLARE _id VARCHAR(29);
DEClARE curid
CURSOR FOR
SELECT concat("_",`id`) -- to set table name
FROM
druk
WHERE
`date` >= _datefrom AND `date` < _dateto;
-- declare NOT FOUND handler
DECLARE CONTINUE HANDLER
FOR NOT FOUND SET finished = 1;
SET #datefrom := _datefrom;
SET #dateto := _dateto;
DROP TEMPORARY TABLE IF EXISTS tableprice;
CREATE TEMPORARY TABLE tableprice (price DECIMAL(8,2));
OPEN curid;
getprice: LOOP
FETCH curid INTO _id;
IF finished = 1 THEN
LEAVE getprice;
END IF;
SET #od = _id;
SET #sql := CONCAT('INSERT INTO tableprice (price) select price from ',_id);
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END LOOP getprice;
CLOSE curid;
-- Diplay temporary table you can use it outside the procedure
SELECT * FROM tableprice;
END
✓
CALL createpricelist(NOW() -INTERVAL 1 DAY,NOW())
| price |
| ----: |
| 2.30 |
| 4.60 |
| 7.90 |
| 0.30 |
| 1.60 |
| 3.90 |
✓
db<>fiddle here