SQL SELECT statement with dynamic column names - mysql

Question

I try to select columns which names are the content of other columns. I'm using MySQL 5.6.
Let's say I have "table1":
+------+------------+------------+---------------+---------------+
| id | val_int1 | val_int2 | val_string1 | val_string2 |
+------+------------+------------+---------------+---------------+
| 1 | 70 | 88 | xxx | yyy |
+------+------------+------------+---------------+---------------+
And "table2":
+------+--------+----------+
| id | type | ref_id |
+------+--------+----------+
| 10 | i1 | 1 |
| 20 | s2 | 1 |
+------+--------+----------+
What I want to do is: join table1 and table2, the table2.type field contains the name of the column from table1 which I want to select. And then there's the problem that the type field only contains abbreviations which I have to extend.
This ends up in the following SQL statement:
SELECT
t1.id,
IF(t2.type REGEXP 'i[0-9]+', REPLACE(t2.type, 'i', 'val_int'), REPLACE(t2.type, 's', 'val_string'))
FROM
table1 t1, table2 t2
WHERE
t1.id = t2.ref_id AND t1.id = 1
The result is that the REPLACE functions return val_int1 and val_string2 as fixed strings and not handle it as column names.
What I really expect is:
+-----+-------+
| 1 | 70 |
| 1 | yyy |
+-----+-------+

Answer 1

You need some sort of case expression:
select t1.id,
(case when t2.type = 'i1' then cast(val_int_1 as varchar(255))
when t2.type = 'i2' then cast(val_int_2 as varchar(255))
when t2.type = 's1' then val_string_1
when t2.type = 's2' then val_string_2
end) as val
from table1 t1 cross join
table2 t2;
You are likely to complain "oh, I have so many columns". Basically, too bad. You have a poor database design. You are trying to do a partial match on strings and column names. Even a dynamic SQL solution is not very feasible.

Answer 2

Using case expression, this is how i would solve this:
DECLARE #table1 TABLE
(
id INT,
val_int1 INT,
val_int2 INT,
val_string1 NVARCHAR(100),
val_string2 NVARCHAR(100)
)
INSERT INTO #table1 VALUES
(1,70,88,'xxx','yyy')
DECLARE #table2 TABLE
(
id INT,
type NVARCHAR(MAX),
ref_id INT
)
INSERT INTO #table2 VALUES
(10,'i1',1),
(20,'s2',1)
SELECT
id,
CASE WHEN type = 'i1' THEN CAST((SELECT TOP 1 val_int1 FROM #table1) AS NVARCHAR(100)) ELSE
CASE WHEN type = 'i2' THEN CAST((SELECT TOP 1 val_int2 FROM #table1) AS NVARCHAR(100)) ELSE
CASE WHEN type = 's1' THEN (SELECT TOP 1 val_string1 FROM #table1) ELSE
(SELECT TOP 1 val_string2 FROM #table1) END END END
FROM #table2 t2
OUTPUT:
10 70
20 yyy