updating a table with a column from another table in SQL - sql

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

This script assumes there is only one record per "Var" value in table1.
--insert all from table1
insert into newtable (var,number)
select var,cur_number from table1 t1
--insert from table2 where new_number < cur_number
insert into newtable (var,number)
select t2.var, t2.new_number
from table2 t2
inner join table1 t1 on t1.var = t2.var and t2.new_number < t1.cur_number

Answer 2

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.

Answer 3

If you know for a fact that you do not have duplicate values in those 2 tables, then you can use UNION ALL instead of UNION
select * from table1 t1
UNION ALL
select* from table2 t2
where t2.new_number < t1.cur_number
By duplicate values, I mean something like this:
table1:
var cur_number
A 8
table2:
var new_number
A 8
The difference is that UNION ALL is faster than UNION, due to the fact that UNION eliminates duplicates from the resultset by using SELECT DISTINCT. In the case you have duplicates, UNION is the best in my experience.

Answer 4

Why not use Union? * SQLFIDDLE
select * from first
union
select s.var,s.new_number from second s
join first f
on s.var = f.var
join (
select f.var, max(f.cur_number) maxn
from first f
group by f.var) as t
on t.var = s.var
where s.new_number < t.maxn
order by var asc
;
results
VAR CUR_NUMBER
A 10
B 8
A 2
B 4
B 6