Question

I need to create a new table from data I extract from the following two tables:

First table:

Var     cur_number
-------------------
 A        10
 B         8

Second table:

Var    new_number
-------------------
 A          2
 A         11
 B          4
 B          6

The new table should contain a ‘Var’ column and a ‘Number’ column, where for each variable there will be one line with its cur_number, and the rest of the lines will contain the numbers from second table’s new_number columns, where the new_number < cur_number.
For example, in the example shown above, for A there will be one line with 10 (its cur_number) and one line with ‘2’ (since 2<10, however 11>10).

in my example the new table will be:

Var         Number
 A            10
 A            2
 B            8
 B            4
 B            6

The database is very large, and the running time is crucial so I cannot use UNION on the two tables…

Answer 1

Try this:

SELECT var, cur_number
FROM FirstTable
UNION
SELECT t2.var, t2.new_number
FROM Firsttable t1
INNER JOIN SecondTable t2 ON t2.new_number < t1.cur_number;

Update: If you are using SQL Server 2008 or above, you can use MERGE to merge the two tables into one like so:

MERGE INTO FirstTable AS TGT
USING SecondTable AS SRC
  ON  SRC.new_number >= TGT.cur_number
WHEN NOT MATCHED THEN
  INSERT (var, cur_number)
  VALUES (SRC.var, SRC.new_number);

SQL Fiddle Demo

This will merge the two tables values into the first table. The first table will contains:

VAR    CUR_NUMBER
A          10
B          8
A          2
B          4
B          6

Note that: When using MERGE:

• You have to terminate the MERGE statement with a semicolon, It is mandatory.
• You can’t INSERT inside WHEN MATCHED thats why I used the reverse condition >= in the WHEN NOT MATCHED.