Answer simple quiz at the end of the blog post and –
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Finding un-matching Records
Often time we want to find records in one table that have no matching key in another table. This is common for things like finding products that have never sold, or students who did not re-enroll. Something we were expecting is missing. Records in one table were expecting some related activity in another table and did not find them. There are many ways to find these records.
Basic Subquery
We have probably all heard that subqueries should be avoided if there is a better solution. Often times basic subqueries are used where a simple “Unmatched Records Query” could have been used.
Let’s start off with the subquery example. Looking at the Location table in the figure below we see all the data. In fact, this table does not allow nulls for the LocationID field. Looking further, there are no nulls anywhere in the Location table. So does this mean that all locations have at least one employee? It’s possible that some of these locations might be new and have not employees working there yet.
How can we find the location with no employees. We can use a subquery. For example we can use the following query to find all locations of the JProCo database that have now employee working there.
SELECT *
FROM Location
WHERE LocationID
NOT IN (SELECT DISTINCT LocationID
FROM Employee
WHERE LocationID IS NOT NULL)
Unmatched Records Queries
If you wanted to find all locations with no employees, you could run an “Unmatched Records Query”. Let’s build this piece by piece. In this case, we have to join the Location table with the Employee table to determine the location that has no employees. What type of join will tell us this? Since nulls don’t map through a join, the INNER JOIN drops the record from the result set and we won’t see Chicago. The outer join will show both the matches and the unmatched records, so we see every location. In the figure below we get all location even if there are no employees.
Notice Seattle is listed many times but Chicago is listed once with no employees found. A NULL appears in the fields from the Employee table for Chicago. With the Location table on the left and the NULL on the right, we have part of an unmatched records query. To find just the records that don’t match, we look for null records on the table that the outer join does not favor. In this case, it’s the Employee table.
The outer join will show us the unmatched records with null location details if you set the WHERE clause to look for nulls on a field in the non-dominant table. Unmatched record queries use SQL to return a result set displaying only the unmatched records between the two tables.
When our query criterion specifies NULL, only Chicago shows up in our result set. By doing a LEFT OUTER JOIN and using a NULL value from the Employee table (or “RIGHT” table) as our search condition criteria, our unmatched records query shows us one record.
Note: If you want to setup the sample JProCo database on your system you can watch this video.
Question 5
Q 5) Which of the following queries will show all location that have no employees?
- SELECT P.EmployeeID, T.[Name]
FROM Employee P LEFT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE T.TerritoryID IS NULL - SELECT P.EmployeeID, T.[Name]
FROM Employee P LEFT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL - SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE T.TerritoryID IS NULL - SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL
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Reference: Pinal Dave (https://blog.sqlauthority.com)
135 Comments. Leave new
Answer for question 5:
Above all option will give error, since the above all option join with common column TerritoryID which is not there in employee table nor in location table
INDIA
Brijesh
Correct , I have not looked at the table columns .It definitely produce an error.
None of option is correct.
The correct answer is option 4. that is
SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL
Here we have used right outer join it means it will bring all records from Location Table and will fatch NULL wherever TerritoryID is NULL for Employee table. So we can match records from Employee table which have TerritoryID as null.
From
INDIA
AHMEDABAD
The correct option is 4.
SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL
Gets all locations that have no employees. The where condition will list all the territoryID from right table(i.e. Employee Table) which contains null values
By
Yasodha.N (India)
Option No. 4 is correct:
SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL
Country- India
OPTION 2 & 4 is Right
2. SELECT P.EmployeeID, T.[Name]
FROM Employee P LEFT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL
4..SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL
Ans:
4. SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL
This queries will show all location that have no employees.
Partha
India
Hi,
4th is Correct:
SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL
I am from India
I think option 4 is the correct answer
SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL
Country- India
Correct Answer is option : 4
SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL
India
The Correct Answer is Option-4
Thanks
Narendra (India)
Correct answer is option 4.
4. SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL
Country – India
Correct option is 4.
SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL
India
Shilpa Sharma
4. SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL;
Here right outer join with null give the value of Location which is not having employee
Ghanshyam
Bangalore, India
The 1, 2 3 are wrong
Bocs
1. will give the employees list whose Territory ID not in Location table
and employees whose territory ID is null in employee table
2. will give the employees list whose territory ID is null in employee table
3. will give the records which having territoryID is null in location table
(If the location table permits null values to the TerritoryID column)
So option 4 is the correct answer
4. which will Give list of location which have no employees
and If the location table permits null values to the TerritoryID column
those rows also will include.
Govindaraj P bangalore India.
The correct option is 4.
SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL
Gets all locations that have no employees. The where condition will list all the territoryID from right table(i.e. Employee Table) which contains null values
From
Malay Shah
Ahmedabad (India)
The correct option is 4.
When we apply right join then the result is all row from right table which is location in our case and respective match records from the left table and null in case of no match.
And applying the where cause of null (P.TerritoryID IS NULL) will only select result having employees with location null
Karan
India
Option 4 is correct
SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL
Country INDIA
Answer is Option# 4.
SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL
Soumya Ranjan Das,
Bhubaneswar, India
The answer is:
SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL
First two options does not work because, it is LEFT OUTER JOIN. If we do this, in the first step itself it does not show the location which does not have employee.
Thirst option does not work because, it is checking for ‘T.TerritoryID IS NULL’ which is can not be null.
So the fourth option will meet the requirement.
Country: India
Hi Pinal,
Correct Answer is Option 4
SELECT P.EmployeeID, T.[Name]
FROM Employee P RIGHT OUTER JOIN Location T
ON T.TerritoryID = P.TerritoryID
WHERE P.TerritoryID IS NULL
Thanks,
Fazal(India)