Concatenating tablenames and columns in joined SQL query - mysql

Question

How can i use the content of switches.address as a table-name, and switches.pin as a column-name to perform some sort of joined query on "switches" that that gives me (in the below case) the value of PIN3 from 0x68?
something like
SELECT name, state FROM switches
with 0x68.PIN3 as state
CREATE TABLE switches (
name varchar(20), address varchar(20), pin varchar(20));
INSERT INTO switches VALUES
("Lights_Kitchen", "0x68", "PIN3");
| name | address | pin |
+----------------+---------+------+
| Lights_Kitchen | 0x68 | PIN3 |
CREATE TABLE `0x68` (
PIN1 INT, PIN2 INT, PIN3 INT, PIN4 INT);
INSERT INTO `0x68` VALUES
(0,0,1,0)
| PIN1 | PIN2 | PIN3 | PIN4 |
+------+------+------+------+
| 0 | 0 | 1 | 0 |
and so on..

Answer 1

I'm not entirely sure about your condition but you can try PREPARED STATEMENTS. Perhaps something like this:
SET #tbl = NULL;
SET #col = NULL;
SET #sql = NULL;
SELECT address INTO #tbl FROM switches WHERE address='0x68';
SELECT pin INTO #col FROM switches WHERE address='0x68';
SELECT #tbl, #col;
SET #sql := CONCAT('SELECT ',#col,' FROM `',#tbl,'`;');
SELECT #sql;
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE stmt;
There are three variables #tbl, #col & #sql here and each are quite self-explanatory. The processes are:
Set all variables with NULL value: just in case that the variables has been used and still holds the last value being set.
Assigning variables with table & column value that need to be used in the final query.
Setting #sql variable with a generated query concatenated with the table and column variables.
Prepare, execute and deallocate the statement.
P/S: Those SELECT #tbl, #col and SELECT #sql query in between are just for checking for what has been set in the variables and not required in the final query structure.
Demo fiddle