Friday, March 9, 2012

JOIN without excluding non-joined rows?

Good morning, folks. I'll get right to the point.

I have an MSSQL db with 3 tables. Table A holds users' data with logins and such. Table B holds workstations with machine names, hardware info, etc. Lastly, Table C holds record IDs from A and B in order to keep track of workstations paired to users. Some workstations don't have users yet, some users don't have workstations yet.

HOWEVER, I need to display all users WITH their workstations, but that same result set needs to include users which may not have a workstation associated with them

My current join query works beautifully, but does NOT include users with no workstations associated with them per Table C.

Table A is lan_user
id_user user_name user_login

Table B is lan_ws
id_ws ws_name ws_model operating_system

Table C is lan_userws_rel (as in, "relationships")
id_userws_rel ws_id user_id

select lan_user.*, lan_userws_rel.*, lan_ws.* from lan_user INNER JOIN lan_userws_rel ON lan_user.id_user = lan_userws_rel.user_id INNER JOIN lan_ws ON lan_ws.id_ws-lan_userws_rel.ws_id

Some additional information:
Workstations can have multiple users and vice versa, but I only really need one pairing in my result set - doesn't matter which one.You should use LEFT JOIN but be aware when you will use a WHERE-clause on the 'not present' items
Info from books online (the guide in troubled moments!)

Joins can be categorized as:

Inner joins (the typical join operation, which uses some comparison operator like = or <>). These include equi-joins and natural joins.

Inner joins use a comparison operator to match rows from two tables based on the values in common columns from each table. For example, retrieving all rows where the student identification number is the same in both the students and courses tables.

Outer joins. Outer joins can be a left, a right, or full outer join.

Outer joins are specified with one of the following sets of keywords when they are specified in the FROM clause:

LEFT JOIN or LEFT OUTER JOIN

The result set of a left outer join includes all the rows from the left table specified in the LEFT OUTER clause, not just the ones in which the joined columns match. When a row in the left table has no matching rows in the right table, the associated result set row contains null values for all select list columns coming from the right table.

RIGHT JOIN or RIGHT OUTER JOIN

A right outer join is the reverse of a left outer join. All rows from the right table are returned. Null values are returned for the left table any time a right table row has no matching row in the left table.

FULL JOIN or FULL OUTER JOIN

A full outer join returns all rows in both the left and right tables. Any time a row has no match in the other table, the select list columns from the other table contain null values. When there is a match between the tables, the entire result set row contains data values from the base tables.

|||{ :-O

Sweet mother of god. LEFT JOIN did essentially just what I needed.

This is proof that I have so, so much to learn about SQL. For the record, I really did look into this before posting, but I sometimes get lost in the examples given in my books.

THANK YOU.

Here was my final query:

select lan_user.*, lan_userws_rel.*, lan_ws.*
from lan_user
LEFT JOIN lan_userws_rel on lan_user.id_user = lan_userws_rel.user_id
LEFT JOIN lan_ws on lan_ws.id_ws = lan_userws_rel.ws_id
ORDER BY id_user

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