SELECTing the column names for another SELECT - possible? - mysql

Question

I have a table t1 with a 'foo' column, and records containing 'bar' and 'baz' in column 'foo'. I also have a table t2 with columns 'bar', 'baz' and 'quux'. I want to do something like the following:
SELECT (SELECT foo from t1) FROM t2;
that is, get the two column names from t1 and query those columns of t2. If I try to do this with MonetDB, I get an error message:
cardinality violation (2>1)
so,
is there some other way to do this with MonetDB?
is this possible in MySQL? other DBMSes?
Example queries (or non-query directives followed by a query) are welcome.

Answer 1

first you need to select the columns into a variable and then use that variable to get tables
something like the following (it is not the actual implementation)
declare #colname varchar(250)
select #colname=foo from t1
select #colname from t2
this may be help.

Answer 2

This is how you can accomplish it in SQL Server
-- Create #T1 which contains names of columns and #T2 which contains the actual columns
CREATE TABLE #T1(colname nvarchar(max))
INSERT INTO #T1 (colname) VALUES ('b')
INSERT INTO #T1 (colname) VALUES ('c')
CREATE TABLE #T2(a int IDENTITY(1,1), b DATETIME DEFAULT GETDATE(), c NVARCHAR(max) DEFAULT 'blah')
INSERT INTO #T2 DEFAULT VALUES
INSERT INTO #T2 DEFAULT VALUES
INSERT INTO #T2 DEFAULT VALUES
--#dSQL will contain the actual SQL string to be executed
DECLARE #dSQL nvarchar(max) = ' '
SELECT #dsql = CONCAT(#dsql,'[',colname,'],') FROM #T1
SELECT #dsql = CONCAT('SELECT',LEFT(#dsql,LEN(#dsql)-1),' FROM #T2')
--You can see the SQL query being executed
PRINT #dsql
--Actually execute it
exec sp_executesql #dsql

Answer 3

MS Sql dynamic sql example.
create table t1(foo varchar(100))
insert t1(foo)
values('bar,baz');
create table t2(bar int, baz int)
insert t2 (bar, baz)
values
(1,100),
(3,300);
declare #cmd varchar(max);
select #cmd= 'select '+ (select top(1) foo from t1) + ' from t2';
EXEC (#cmd);
Result
bar baz
1 1 100
2 3 300
Or may be foo contains column names in different rows, not exactly clear from your question.
create table t1(foo varchar(100))
insert t1(foo)
values('bar'),('baz');
create table t2(bar int, baz int)
insert t2 (bar, baz)
values
(1,100),
(3,300);
declare #cmd varchar(max);
select #cmd= 'select '+ stuff((select ','+ foo from t1 for xml path('')),1,1,'') + ' from t2';
EXEC (#cmd);