The IF EXISTS and NOT EXISTS commands in T-SQL are covered in depth in this article. When comparing data sets using subqueries, it also illustrates why EXISTS should be preferred over IN and NOT EXISTS over NOT IN. If you don't know, EXISTS is a logical operator in SQL that is used to check if rows in a database exist. If the subquery produces one or more records, it returns TRUE. The NOT EXISTS operator in SQL is the polar opposite of the EXISTS operator, and it is fulfilled if the subquery returns no results.
Using EXISTS in SQL
SELECT column_name FROM Table_Name WHERE EXISTS (SELECT column_name FROM Table_Name WHERE condition);
Let's understand this by example using a customer database.
you
guys can feel free to create your own tables. create two tables. table 1
should have a one-to-many relationship with table 2. I have created two
sample tables Customer and Orders. A customer can have multiple Orders,
thus, creating a one-to-many relationship.
Now, Let's
imagine we want to start a remarketing campaign and require a list of
consumers who have made at least one purchase. So, with SQL, how do you
see whether a row exists? We'll use the following query to do this:
SELECT * FROM Sales.Customer c WHERE EXISTS (SELECT * FROM Sales.[Order] o WHERE o.CustomerId = o.OrderId)
The query returns rows from the Customer table whose CustomerID columns are the same as the OrderID fields in the Order table.
Using NOT EXISTS in SQL
The
NOT EXISTS condition in SQL Server is made up of two logical operators:
EXISTS (which was previously discussed) and NOT (which is used to
negate a Boolean input).
What is the difference between the SQL terms EXISTS and NOT EXISTS?
NOT
EXISTS, unlike EXISTS, returns TRUE if the subquery's result contains
no records. If a single entry in a table fits the subquery, the NOT
EXISTS function returns FALSE, and the subquery's execution is halted.
NOT EXISTS, in plain English, allows you to find records that don't
match the subquery.
SYNTAX
SELECT column_name FROM Table_Name WHERE NOT EXISTS (SELECT column_name FROM Table_Name WHERE condition);
NOT
EXISTS is used in the WHERE clause with a subquery to see if the result
of the subquery is TRUE or FALSE. The outer select statement's rows are
subsequently narrowed down using the Boolean value.
To
put it another way, the NOT EXISTS subquery checks every entry from the
outer query, returns TRUE or FALSE, and then passes the value to the
outer query to use. In plain English, SQL NOT EXISTS returns all entries
that do not fulfill the EXISTS condition.
SQL Exists and Not Exists Example
We'll
query the Customer table to locate entries where the CustomerID doesn't
exist in the Order table to show how NOT EXISTS works in SQL Server. As
a result, we'll obtain a list of clients who haven't placed any orders
yet if we use NOT EXISTS.
SELECT * FROM Sales.Customer c WHERE NOT EXISTS IN (SELECT * FROM Sales.[Order] o WHERE o.CustomerId = o.OrderId)
Result
Conclusion
Because
SQL is a logical language, everything works fairly simply. Its logical
operators examine the situation and return a Boolean value. In some
situations, these logical operators can be used interchangeably, and the
user can choose which one to employ. We gave a complete review and
comparison of SQL EXISTS, NOT EXISTS, IN in this post so that you may
make the best decisions possible in your work.
Other related SQL queries, Interview questions, and articles:
Thanks for reading this article, if you like this SQL EXISTS tutorial, then please share it with your friends and colleagues. If you have any questions or feedback, then please drop a note.
P.S. - If you are interested in learning Database and SQL and looking for some free resources to start your journey, then you can also take a look at the Introduction to Databases and SQL Querying free course on Udemy to kick-start your learning.
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Thanks for reading this article, if you like this SQL EXISTS tutorial, then please share it with your friends and colleagues. If you have any questions or feedback, then please drop a note.
P.S. - If you are interested in learning Database and SQL and looking for some free resources to start your journey, then you can also take a look at the Introduction to Databases and SQL Querying free course on Udemy to kick-start your learning.
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