What is Exists in SQL Server? How to use NOT Exists in SQL Server how? Let's find out with Quantrimang.com!
SQL Server is knowledge that every programmer needs to know. When using SQL Server, you have to get familiar with many functions or clauses, Exists is one of them.
In SQL Server (Transact-SQL) the EXISTS condition is used to combine with internal queries (subquery). The condition is met if the internal query returns at least 1 row. This condition can be used in SELECT, INSERT, UPDATE or DELETE commands.
EXISTS clause syntax in SQL Server
WHERE EXISTS ( subquery);
Variable name or variable value
subquery
Subquery – the internal query is a SELECT statement. If this query returns at least 1 record in the result set then the EXISTS clause evaluates to true and the EXISTS condition is met. If the internal query returns no records, the EXISTS clause evaluates to false and the EXISTS condition is not met.
Note
Using the EXISTS condition is ineffective because the internal query RE-RUN on each row in the table in the external query. There are more efficient ways without using the EXISTS condition.
For example – with the SELECT command
The SELECT command is used with the EXISTS condition as shown below.
SELECT *
FROM nhanvien
WHERE EXISTS (SELECT *
FROM danhba
WHERE nhanvien.ho = danhba.ho
AND nhanvien.t
en = danhba.ten);
This example will return all records from the nhanvien table when there is at least 1 record in the nhanba table whose first and last names match the first and last names in the nhanvien table.
Example – SELECT command uses NOT EXISTS
The EXISTS condition can be combined with the NOT operator.
SELECT *
FROM nhanvien
WHERE NOT EXISTS (SELECT *
FROM danhba
WHERE nhanvien.ho = danhba.ho
AND nhanvien
.ten = danhba.ten);
The returned result is all records in the nhanvien table if there are no first and last names records in the nhanba table that match the first and last names in the nhanvien table.
Example – INSERT command
Here is an example of using the INSERT command with the EXISTS condition
INSERT INTOdanhba
(danhba_id, danhba_ten)
SELECT nhacung_id, nhacung_ten
FROM nhacung
WHERE EXISTS (SELECT *
FROM donhang
WHERE nhacung.n
hacung_id = donhang.nhacung_id);
Example – UPDATE command
Below is an example of the UPDATE command using the EXISTS condition.
UPDATE nhacung
SET nhacung_ten = (SELECT khachhang.ten
FROM khachhang
WHERE khachhang.khachhang_id = nhacung.nhacung_id)
WHERE EXISTS (SELECT khachhang.ten
FROM khachhang
WHERE khachhan
g.khachhang_id = nhacung.nhacung_id);
Example – DELETE command
The DELETE command can also be used with the EXISTS condition as shown below.
DELETE FROM danhba
WHERE EXITS (SELECT *
FROM nhanvien
WHERE nhanvi
en.ho = danhba.ho );
How to use NOT with EXIST condition in SQL Server
The NOT command is used to extract WHERE condition records provided by the user that are NOT TRUE or FALSE.
This example retrieves STUDENT_NAME from the STUDENT table, for the given condition. For example: department.student_id=student.student_id is FALSE.
Result:
If you see that both tables are clear, you will know that both given names, located in STUDENT_ID are not mentioned in the DEPARTMENT table, so it is the result.
If we do not use the NOT command here, it will result in Neena and Johny.
Note: Although STUDENT_ID for STUDENT_NAME Kashish is currently in the DEPARTMENT table. It is not yet shown in the results here.
EXISTS vs. JOIN
The SQL Server EXISTS operator returns TRUE or FALSE, and the JOIN clause returns rows from another table.
You use the EXISTS operator to check whether a subquery returns any rows and short periods as soon as it runs. On the other hand, you use JOIN to form a result set by combining it with columns from the related table.
In fact, you use EXISTS when you need to check the existence of rows from a related table without returning data from them.
Examples of EXISTS and IN
The following command uses the IN operator to find customer orders from San Jose:
SELECT
*
FROM
sales.orders
WHERE
customer_id IN (
SELECT
customer_id
FROM
sales.customers
WHERE
city = 'San Jose'
)
ORDER BY
customer_id,
order_date;
The following command uses the EXISTS operator, returning the same result:
SELECT
*
FROM
sales.orders o
WHERE
EXISTS (
SELECT
customer_id
FROM
sales.customers c
WHERE
o.customer_id = c.customer_id
AND city = 'San Jose'
)
ORDER BY
o.customer_id,
order_date;
Frequently asked questions about SQL Exists
Where is NOT EXISTS SQL used?
NOT EXISTS SQL means the subquery does not return any results. It is used to limit the number of rows returned by a SELECT statement. In this server, it checks the subquery for row existence. If yes, it returns true, otherwise it returns false.
How to avoid NOT EXISTS in SQL?
To fix NOT EXISTS error in SQL Server, make sure you are searching on an indexed page. Make sure there is no data manipulation in the necessary columns. Now replace No EXISTS with a left outer join function, which will work better on large data sets.
How to check if there is a NOT EXISTS record in SQL?
There are different ways to check if a NOT EXIST record is present in SQL. Using the IF statement in EXISTS is a typical example.
NOT in the SQL operator is used when the user is accessing a column, with no entries in the table. The Customers table contains records of all other lists and related transactions.
Above are the basic things you need to know about EXISTS in SQL Server. This is a useful conditional command that you should definitely know when learning about SQL Server. Hopefully the basic knowledge above will help you program with SQL Server more effectively.
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