Tag - DBA

DELETE and UPDATE CASCADE in SQL Server foreign key
Jul 20, 2020

In this article, we will review on DELETE AND UPDATE CASCADE rules in SQL Server foreign key with different examples. DELETE CASCADE: When we create a foreign key using this option, it deletes the referencing rows in the child table when the referenced row is deleted in the parent table which has a primary key. UPDATE CASCADE: When we create a foreign key using UPDATE CASCADE the referencing rows are updated in the child table when the referenced row is updated in the parent table which has a primary key. We will be discussing the following topics in this article: Creating DELETE CASCADE and UPDATE CASCADE rule in a foreign key using T-SQL script Triggers on a table with DELETE or UPDATE cascading foreign key Let us see how to create a foreign key with DELETE and UPDATE CASCADE rules along with few examples.   Creating a foreign key with DELETE and UPDATE CASCADE rules Please refer to the below T-SQL script which creates a parent, child table and a foreign key on the child table with DELETE CASCADE rule.   Insert some sample data using below T-SQL script.   Now, Check Records.   Now I deleted a row in the parent table with CountryID =1 which also deletes the rows in the child table which has CountryID =1.   Please refer to the below T-SQL script to create a foreign key with UPDATE CASCADE rule.   Now update CountryID in the Countries for a row which also updates the referencing rows in the child table States.   Following is the T-SQL script which creates a foreign key with cascade as UPDATE and DELETE rules. To know the update and delete actions in the foreign key, query sys.foreign_keys view. Replace the constraint name in the script.   The below image shows that a DELETE CASCADE action and UPDATE CASCADE action is defined on the foreign key. Let’s move forward and check the behavior of delete and update rules the foreign keys on a child table which acts as parent table to another child table. The below example demonstrates this scenario. In this case, “Countries” is the parent table of the “States” table and the “States” table is the parent table of Cities table.   We will create a foreign key now with cascade as delete rule on States table which references to CountryID in parent table Countries.   Now on the Cities table, create a foreign key without a DELETE CASCADE rule. If we try to delete a record with CountryID = 3, it will throw an error as delete on parent table “Countries” tries to delete the referencing rows in the child table States. But on Cities table, we have a foreign key constraint with no action for delete and the referenced value still exists in the table.   The delete fails at the second foreign key.   When we create the second foreign key with cascade as delete rule then the above delete command runs successfully by deleting records in the child table “States” which in turn deletes records in the second child table “Cities”.   Triggers on a table with delete cascade or update cascade foreign key An instead of an update trigger cannot be created on the table if a foreign key on with UPDATE CASCADE already exists on the table. It throws an error “Cannot create INSTEAD OF DELETE or INSTEAD OF UPDATE TRIGGER ‘trigger name’ on table ‘table name’. This is because the table has a FOREIGN KEY with cascading DELETE or UPDATE.” Similarly, we cannot create INSTEAD OF DELETE trigger on the table when a foreign key CASCADE DELETE rule already exists on the table.   Conclusion In this article, we explored a few examples on DELETE CASCADE and UPDATE CASCADE rules in SQL Server foreign key. In case you have any questions, please feel free to ask in the comment section below.

Table partitioning in SQL
Jun 10, 2020

What is table partitioning in SQL? Table partitioning is a way to divide a large table into smaller, more manageable parts without having to create separate tables for each part. Data in a partitioned table is physically stored in groups of rows called partitions and each partition can be accessed and maintained separately. Partitioning is not visible to end-users, a partitioned table behaves like one logical table when queried. Data in a partitioned table is partitioned based on a single column, the partition column often called the partition key. Only one column can be used as the partition column, but it is possible to use a computed column. The partition scheme maps the logical partitions to physical filegroups. It is possible to map each partition to its own filegroup or all partitions to one filegroup.

Schedule Database backup on SQL Server Express Edition
Sep 20, 2019

Have you ever attempted to set up an automated backup of your SQL Server Express Edition and found that there’s no SQL Server Agent where you can schedule the job which will took a backup of your database. Alas, the world does not end there and you don't need to pay extra bucks just to have the back up via an SQL Agent which is available only to Standard and Enterprise editions. There are many options to automate the backup job which runs on a specific time and does not require manual intervention. Here, we will learn how to do it via SQL Command using batch file and Windows in-build Task Scheduler. Hope, you may find this useful. Create a BAT(batch) file to execute the command to take a backup of Database and save it. echo off :: -------------------------------------------------- :: clear console cls :: -------------------------------------------------- :: Define variables set SERVERNAME=YOUR_SERVER_NAME set DATABASENAME=DATABASE_NAME set MyTime=%TIME: =0% set MyDate=%DATE:~-4%.%DATE:~7,2%.%DATE:~4,2%.%MyTime:~0,2%.%MyTime:~3,2%.%MyTime:~6,2% set FileName=%DATABASENAME%_%MyDate%.bak set BAK_PATH=DIRECTORY_PATH set DEST_FILE=%BAK_PATH%%FileName% :: -------------------------------------------------- :: BACKUP Database sqlcmd -E -S %SERVERNAME% -d master -Q "BACKUP DATABASE [%DATABASENAME%] TO DISK = N'%DEST_FILE%' WITH INIT , NOUNLOAD , NAME = N'%DATABASENAME% backup', NOSKIP , STATS = 10, NOFORMAT" :: -------------------------------------------------- :: Optional Part :: -------------------------------------------------- :: Zip file 7z a -tzip "%DEST_FILE%.zip" "%DEST_FILE%" :: -------------------------------------------------- :: Delete unziped file DEL "%DEST_FILE%"   “SERVERNAME” is the name of SQL Server physical machine. “DATABASENAME” is the database which will be backup. “FileName” sets as a database name and append date which has .bak extension  “BAK_PATH” is the path in which a database backup file will be saved. “DEST_FILE” is use backup path and file name. After defining all the variables database backup will be generated and save as zip file in “DEST_FILE” path and at the end, the unzipped file will be deleted from “DEST_FILE” Now, it's time to schedule this created batch file in #1 Start Menu -> Task Scheduler -> Run as administrator Click on Create Task... from the right bar and configure it with Triggers and Actions