Using query results from one table in the SELECT list for another table

Question

I have 2 tables:

t1

camp_1        col_1
camp_2        col_2

t2

 1      2     3
 2      4     6
 3      6     9
 4      8    12

I want to create a statement to use results from t1 in the SELECT list for t2 and show t2 results.

Example:

Step 1: get results from table 1

select col_name from t1 where campaign_name = 'camp_1'

Result: col_1

Step 2: Use the result ( col_1 ) for t2 and show the corresponding results

select col_1 from t2 

The end results should be

1
2
3
4

Answer 1

Consider to change your schema to something like this:

t1 (unchanged)

camp_1        col_1
camp_2        col_2

t2

group1     col_1      1
group1     col_2      2
group1     col_3      3
group2     col_1      2
group2     col_2      4
group2     col_3      6
group3     col_1      3
group3     col_2      6
group3     col_3      9
group4     col_1      4
group4     col_2      8
group4     col_3     12

You can then select:

SELECT num
FROM t1
  JOIN t2 ON t1. col_name = t2. col_name
WHERE t1.campaign_name = 'camp_1'

Benefits:

• The SQL-Server can optimize your query, which makes it faster
• Use single SQL to query both tables
• You can define more sophisticated statements, by combining conditions for both tables at once

---

If you cannot change your schema, there is still a possibility to select the values without dynamic SQL. You can use the elaboratet feature UNPIVOT - which converts data from several columns into data in additional rows.

WITH unpivot_t2 AS (
    SELECT  *
    FROM   t2
    UNPIVOT  
      (value FOR col_name IN (col_1, col_2, col_3))
    AS unpvt
)
SELECT value
FROM unpivot_t2
JOIN t1 ON t1.col_name = unpivot_t2.col_name
WHERE campaign_name = 'camp_1'

With this DDL:

CREATE TABLE t1 (
    campaign_name varchar(255),
    col_name varchar(255)
)
INSERT INTO t1 VALUES
    ('camp_1', 'col_1'),
    ('camp_2', 'col_2')

CREATE TABLE t2 (
    col_1 varchar(255),
    col_2 varchar(255),
    col_3 varchar(255)
)
INSERT INTO t2 VALUES
    (1,2,3),
    (2,4,6),
    (3,6,9),
    (4,8,12)

Give it a try online!

Answer 2

If you cannot change your schema as slartidan said. Then, first, you have to get your required column name(s) from the table, t1 .

Way One (Using SQL Concatenation)

DECLARE @Columns VARCHAR(MAX) = ''

SELECT @Columns = @Columns + col_name + ','
FROM t1
WHERE campaign_name = 'camp_1'
GROUP BY col_name
SET @Columns = LEFT(@Columns,LEN(@Columns)-1)

Way Two (Using FOR XML PATH):

DECLARE @Columns VARCHAR(MAX) = ''

SELECT @Columns = STUFF((SELECT ',' + col_name
FROM t1
WHERE campaign_name = 'camp_1'
GROUP BY col_name
FOR XML PATH('')) ,1,1,'')

Then you have to execute a raw SQL.

EXEC ('SELECT ' + @Columns + ' FROM t2')

Hope, Then you can get dynamic column values from your desired table, t2 .