Overview of the SQL Delete statement in SQL Server

This article on the SQL Delete is a part of the SQL essential series on key statements, functions and operations in SQL Server.

To remove a row from a tableis accomplished through a Data Manipulation Language, aka DML statement, using the delete keyword.The SQL delete operation is by far the simplestof all the DML commands.On execution of the delete command, we don’t have to worry about gettingany form of data from the table,and we don’t have to worry aboutworking with any data that we get back from the table(s).We just simply tell the databaseto delete a specific record,and it either does or it doesn’t.It’s that simple.

First, let’s quickly reviewwhat an SQL delete statement looks like. We need to tell the database and table from where it should delete the data.It’s a good idea to add a conditionclause to set the scope of data deletion.Otherwise, it will delete everything in the table.

Let’s take a look at our tableand removing some records.

How to delete rows with no where clause

The following example deletes all rows from the Person.Person the table in the AdventureWorks2014 database. There is no restriction enforced on the SQL delete statement using a WHERE clause.

USE Adventureworks2014; GO DELETE FROM [Person].[Person]; How to delete rows with where clause The following example deletes rows from the [Person].[Person] table in the AdventureWorks2014 database in which the value in the businessEntityID column is greater than 30,,000 USE Adventureworks2014; GO DELETE FROM [Person].[Person] WHERE businessEntityID > 30000;

Note: An unfortunate mistake that may occur is to accidently run a SQL Delete with no Where clause and inadvertently delete all of your data. To prevent this from happening consider using the Execution guard feature in ApexSQL Complete, to warn against such potentially damaging actions, before you execute them. Learn more: Execution alerts

How to delete rows using Top with where clause

The following example deletes 50 random rows from the Person.Person table in the AdventureWorks2014 database. The value in the BusinessEntityID must be in between 30,000 and 40,000

USE Adventureworks2014; GO DELETE TOP(50) FROM [Person].[Person] WHERE BusinessEntityID between 30000 and 40000

Note: The when the TOP (n) clause is used with the SQL Delete statement and any DML statement (i.e. Select, Insert, Delete and Update), the operation is performed on a random selection of a number of rows specified in the Top clause.

How to delete duplicate rows

In the real-world, we tend to gather datafrom different sources; it’s not uncommonto have duplicate records. One approach to the duplicate problem is first to identify where the duplicates have occurred. And run a select query on those columns.

EATE TABLE tb_spaceused (database_name NVARCHAR(128), database_size VARCHAR(18), [unallocated space] VARCHAR(18), reservedVARCHAR(18), dataVARCHAR(18), index_sizeVARCHAR(18), unusedVARCHAR(18) ); INSERT INTO tb_spaceused EXEC sp_msforeachdb @command1 = "use ? exec sp_spaceused@oneresultset = 1"; SELECT * FROM tb_spaceused order by database_name

The following example uses thePARTITION BYargument to partition the query result set by all the columns of tb_spaceused table. The Row_Number (), a window function, which means it operates over an ordered set. The ORDER BYclause specified in theOVERclause orders the rows in each partition by the entire columns tb_spaceused table.

WITH CTE AS (SELECT *, ROW_NUMBER() OVER(PARTITION BY database_name, database_size, [unallocated space], reserved, data, index_size, unused ORDER BY database_name ) AS Row_Num FROM tb_spaceused) SELECT * FROM CTE WHERE Row_Num <> 1;

Replacing the Select statement with a Delete removes all the duplicates of the table.

WITH CTE AS (SELECT *, ROW_NUMBER() OVER(PARTITION BY database_name, database_size, [unallocated space], reserved, data, index_size, unused ORDER BY database_name ) AS Row_Num FROM tb_spaceused) --SELECT * --FROM CTE --WHERE Row_Num <> 1; DELETE FROM CTE WHERE Row_Num <> 1;

How to delete rows using SQL sub-queries

In the following example, the rows in one table are deleted based on data in another table. In the examples, the rows from the SalesPersonQuotaHistory table are deleted based on the SalesYTD column of the SalesPersontable.

DELETE FROM Sales.SalesPersonQuotaHistory WHERE BusinessEntityID IN ( SELECT BusinessEntityID FROM Sales.SalesPerson WHERE SalesYTD > 4900000.00 ); GO How to delete rows usingSQL Joins

In this section, we will use the SQL Delete statementto delete the data from the Adeventureworks2014 database.Deleting data, at first sight, sound trivial,but once we get into a large database design things might not be same and easy anymore.

In many cases, the tables are related via a primary and foreign key relationship. In the following example, we can see a use of joins to delete the data from the Sales.SalesPersonQuotaHistory.

DELETE sq FROM Sales.SalesPersonQuotaHistory sq INNER JOIN Sales.SalesPerson sp ON sq.BusinessEntityID = sp.BusinessEntityID WHERE sp.SalesYTD > 4500000.00; GO How to delete rows from a remote table using linked servers and OpenQuery

The following example uses the SQL delete statement to delete rows from a remote table using the linked server named, hqdbt01. Then query the remote table using four-part object naming convention to delete the rows from the remote table

DELETE FROM [hqdbt01].AdventureWorks2014.[HumanResources].[Shift] WHERE ShiftID = 2;

The following example, the remote table is queried by specifying the OPENQUERY rowset function along with the delete command.

DELETE OPENQUERY (hqdbt01, 'SELECT * FROM AdventureWorks2014.HumanResources.Department WHERE DepartmentID = 18'); How to delete rows using SSMS

Using the SQL Server Management Studio (SSMS), Graphical User Interface (GUI) to delete rows involves a manual search.In reality, it will be much easier and quicker to delete records with a SQL query.

Let’s go ahead and locate the table to use a SQL delete statement, in this case, table dbo.cities is selected. Now, right-click and choose Edit Top 200 rows . This opt