If you want to keep all the data, and not just the data related to each other, you can use an OUTER join. There are three types of Outer Join: LEFT JOIN, RIGHT JOIN, and FULL JOIN. The differences between them involve which unrelated data they keep – it can be from the first table, from the second, or from both of them.
What is an outer join?
We'll see how this works below with an example. If you want to keep all the data, and not just the data related to each other, you can use an OUTER join. There are three types of Outer Join: LEFT JOIN, RIGHT JOIN, and FULL JOIN.
Is it possible to add a case in the join?
I do not believe that you can do it in the join but you could achieve the same thing using the left join and putting the case logic into the WHERE clause. This creates an inner join on the key and left join where the value on the a table is 2, 3 or 4.
How to switch between two tables in an outer join?
I've got a scenario where I want to switch on two different tables in an outer join. It goes something like this:- select mytable.id, yourtable.id from mytable left outer join (case when mytable.id = 2 then table2 yourtable on table1.id = table2.id else table3 yourtable on table1.id = table3.id end)
What is an example use case for a join of customers?
An example use case would be to produce a report that shows ALL customers and their purchases. That is, show even customers who have not purchased anything. If you do an ordinary join of customers and purchases, the report would show only those customers with at least one purchase. Share Follow edited Jul 9 '09 at 4:28
What does "full outer join" mean?
In SQL Full Outer Join, all rows from both the tables are included. If there are any unmatched rows, it shows NULL values for them.
Why is there only one value for epid 11?
In the following screenshot, you can see that only one NULL value for EmpID 11. It is because EmpID 11 is not available in the Employee table.
How to represent a logical relationship between two tables?
We can represent a logical relationship between two tables using a Venn diagram. In a Venn diagram contains multiple overlapping circles and each circle represents an entity or table. The common area or overlapping area in Venn diagram represents the common values between both tables.
What does "execute this command and view the output" mean?
Execute this command and view the output. It only returns rows that do not match either in Employee or Departments table.
Does EmpID 7 have matching rows?
EmpID 7, 8, 9 exists in the Employee table but not in the Departments table. It does not include any matching rows in the departments table; therefore; we get NULL values for those records
Can you see a few records with null values?
We can see a few records with NULL values as well. Let’s understand this in a better way using a Venn diagram.
Can you add a where clause to SQL?
We can add a WHERE clause with a SQL FULL OUTER JOIN to get rows with no matching data between the both Join tables.
What is a full outer join?
You can think of the FULL OUTER JOIN as the combination of a Left Join and Right Join. It will keep all rows from both tables, and the missing data will be filled in with NULL.
What is the right outside join?
The RIGHT OUTER JOIN, or simply Right Join, will keep the data in the second table that's not related to the first table.
What is a JOIN in SQL?
There are various types of joins, divided into two main categories – INNER joins and OUTER joins.
What does inner join mean in Venn diagram?
The inner join will keep only the information from the two joined tables that is related. If you imagine the two tables as a Venn diagram, the table resulting from an INNER JOIN will be the green highlighted part below where they overlap:
What is the difference between inner and outer join?
The biggest difference between an INNER JOIN and an OUTER JOIN is that the inner join will keep only the information from both tables that's related to each other (in the resulting table). An Outer Join, on the other hand, will also keep information that is not related to the other table in the resulting table.
What happens if you do the same query using right join?
If you do the same query using RIGHT JOIN you would get yet a different result.
What is a left join?
The LEFT OUTER JOIN , or simply Left Join, will keep the unrelated data from the left (the first) table.
Why do I need to use a declared variable?
This allows for stating your left and right-hand side of what you need to compare. This is for supporting an SSRS report where different fields must be linked based on the selection by the user.
What is the meaning of "back up"?
Making statements based on opinion; back them up with references or personal experience.
Why add a second case statement?
A second case statement could be added for the right-hand side if the variable is needed to choose from different fields
How many ways to join based on condition?
There are at least 2 ways to join based on condition. One is faster than the other:
What is a case expression?
A CASE expression returns a value from the THEN portion of the clause. You could use it thusly:
Can you use boolean in a case statement?
I would guess that if you put your CASE statement in your SELECT you would get a boolean '1' or '0' indicating whether the CASE statement evaluated to True or False. Yes, you can. Here is an example.
Can you specify join condition?
You can not specify the join condition as you are doing.. Check the query above that have no error. I have take out the common column up and the right column value will be evaluated on condition.
A Case Statement in a SQL Join
When joining two or more tables, sometimes you end up in a scenario where values you are joining on just don’t quite match up.
The Party Trick of the Century!
I’m guessing that it’s not common knowledge, mostly because I can’t recall ever hearing this, but a CASE statement can be used in the ON portion of a JOIN statement, as shown below:
When Can This be Useful?
There might be a time, and I speak from experience, where this can be a big time saver when updating an existing query. You might have other joins and whatnots going on that you don’t want to mess around with. All you really hope for is to join the table in correctly. This helps get you there!
Final Thoughts
SQL is a powerful and useful tool. There’s definitely more than one way to crack an egg/write code to do what you need it to do. This is just something you might want to add to your toolbox. Until next time, keep on learnin’!
