SQL Server - Join to void nulls - sql

Question

Am sorry I couldn't explain the issue clearly. The actual problem is I have transaction table that contains item transactions such as purchase and sales across various location. I need to find the unit purchase cost of all items across all branches. Now, in a given location, all items may not be purchased. At the same time all items would be purchased in the central warehouse. Means, some items are transferred from Warehouse to locations instead of purchase at the location. In such cases, the unit cost should be picked from the central warehouse purchase data.
Now, I can get the items and purchase cost across each location from the transaction table, given that the item is purchased at the location. My question was, how to fetch the central warehouse price for items that do not have a purchase history in the transaction table and list it along with all other location purchase cost. Why its difficult is, if there is no purchase hist it means i have no item number to search in the central warehouse.
Frankly I do not know how to do this through SQL query in a single go. Hence i did make a master view as first step - containing all branches and items. This is not ideal because the data is so huge as I have around 50 locations and 200K items resulting in 50 x 200k rows. However, it served the purpose of acting as a location-item master.
Second step, I made central warehouse master with item and purchase cost at the warehouse.
Thirdly, queried transaction table to fetch items that has no purchase at specific locations. These item id was linked to location-item master and used a case statement to get, if purchase cost is null then get the cost from warehouse.
Thank you for pointing out the mistakes and for introducing COALESCE.
Table (Tab1) is as below:
┌─────────┐
│ TabCol1 │
├─────────┤
│ 01 │
│ 02 │
│ 03 │
│ 04 │
│ 05 │
└─────────┘
I have a table (Tab2 ) with two columns:
┌──────┬──────┐
│ Col1 │ Col2 │
├──────┼──────┤
│ 1111 │ 01 │
│ 1111 │ 02 │
│ 1111 │ 03 │
└──────┴──────┘
If we join the above table we get:
┌─────────┬──────┬──────┐
│ TabCol1 │ Col1 │ Col2 │
├─────────┼──────┼──────┤
│ 01 │ 1111 │ 01 │
│ 02 │ 1111 │ 02 │
│ 03 │ 1111 │ 03 │
│ 04 │ NULL │ NULL │
│ 05 │ NULL │ NULL │
└─────────┴──────┴──────┘
What I need is, instead of NULL, I must get 1111:
┌─────────┬──────┬──────┐
│ TabCol1 │ Col1 │ Col2 │
├─────────┼──────┼──────┤
│ 01 │ 1111 │ 01 │
│ 02 │ 1111 │ 02 │
│ 03 │ 1111 │ 03 │
│ 04 │ 1111 │ 04 │
│ 05 │ 1111 │ 05 │
└─────────┴──────┴──────┘
In other words, I need to make a master table, with all COL1 filled to avoid NULL.

Answer 1

What you are trying to achieve makes completely no sense to me, but there's one way to get that result:
select T1.TabCol1,
coalesce(T2.Col1, '1111'),
coalesce(T2.Col2, T1.TabCol1)
from Tab1 T1 left join Tab2 T2 on T1.TabCol1 = T2.Col2

Answer 2

You can replace NULLs with whatever you want:
SELECT TabCol1, ISNULL(Col1, '1111') AS Col1, ISNULL(Col2, TabCol1) AS Col2
FROM Tab2
LEFT JOIN Tab1 ON Tab1.TabCol1 = Tab2.Col2
Note that ISNULL only works in SQL Server. Alternatively, you can use COALESCE which is supported by most databases:
SELECT TabCol1, COALESCE(Col1, '1111') AS Col1, COALESCE(Col2, TabCol1) AS Col2
FROM Tab2
LEFT JOIN Tab1 ON Tab1.TabCol1 = Tab2.Col2