SQL Server. Lecture3 Cascading referential integrity constraint

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1 SQL Server Lecture3 Cascading referential integrity constraint

3 insert into tblperson values Msg 547, Level 16, State 0, Line 1 The INSERT statement conflicted with the FOREIGN KEY constraint "FK_tblPerson_GenderID". The conflict occurred in database "Simple", table "dbo.tblgender", column 'ID'. The statement has been terminated.

9 However, you have the following options when setting up Cascading referential integrity constraint 1. No Action: This is the default behavior. No Action specifies that if an attempt is made to delete or update a row with a key referenced by foreign keys in existing rows in other tables, an error is raised and the DELETE or UPDATE is rolled back. 2. Cascade: Specifies that if an attempt is made to delete or update a row with a key referenced by foreign keys in existing rows in other tables, all rows containing those foreign keys are also deleted or updated.

15 3. Set NULL: Specifies that if an attempt is made to delete or update a row with a key referenced by foreign keys in existing rows in other tables, all rows containing those foreign keys are set to NULL.

19 4. Set Default: Specifies that if an attempt is made to delete or update a row with a key referenced by foreign keys in existing rows in other tables, all rows containing those foreign keys are set to default values.

24 SQL Server Lecture3 Identity column in SQL Server

26 If a column is marked as an identity column, then the values for this column are automatically generated, when you insert a new row into the table. The following, create table statement marks PersonId as an identity column with seed = 1 and Identity Increment = 1. Seed and Increment values are optional. If you don't specify the identity and seed they both default to 1.

27 How to create Identity column? In graphical:

32 SQL Queiry: Create Table tblperson1 ( PersonId int Identity(1,1) Primary Key, Name nvarchar(20) )

34 Insert into tblperson1 values ('Sam') Insert into tblperson1 values ('Sara')

43 How to insert the same delete identity column? SET Identity_Insert tblperson1 ON Insert into tblperson(id, Name) values(2, Sara') SET Identity_Insert tblperson1 OFF

46 How to reset identity? If you have deleted all the rows in a table, and you want to reset the identity column value, use DBCC CHECKIDENT command. This command will reset PersonId identity column. DBCC CHECKIDENT(tblPerson1, RESEED, 0)

47 How to get the last generated identity column value in SQL Server

48 From the previous session, we understood that identity column values are auto generated. There are several ways in sql server, to retrieve the last identity value that is generated. The most common way is to use SCOPE_IDENTITY() built in function. And also can use IDENT_CURRENT('TableName')

49 SCOPE_IDENTITY() returns the last identity value that is created in the same session (Connection) and in the same scope (in the same Stored procedure, function, trigger). returns the last identity value that is created in the same session without any consideration to the scope IDENT_CURRENT(TableName) returns the last identity value created for a specific table across any session and any scope.

64 SQL Server Lecture 4 Unique Key Constraint and Select Statement

65 Unique Key Constraint

66 What is a unique key constraint? When do we use unique key constraint over primary key constraint? Difference between unique key constraint and primary key constraint.

67 We use UNIQUE constraint to enforce uniqueness of a column i.e. the column shouldn't allow any duplicate values. We can add a Unique constraint through the designer or by using query.

68 How to create a Unique Key in design From the table tblperson consider that we want to add a unique key constraint on the Column

69 1. Right-click on the table and select Design 2. Right-click on the column, and select Indexes/Keys...

70 3. Click Add

71 4. For Columns, select the column name you want to be unique.

72 5. For Type, choose Unique Key. 6. For the Name choose the constraint s Name.

73 6. Click Close, Save the table.

74 To create the unique key using a query: Alter Table Table_Name Add Constraint Constraint_Name Unique(Column_Name)

80 Insert Data to column that have a unique key

83 Both primary key and unique key are used to enforce the uniqueness of a column. So, when do you choose one over the other? A table can have only one primary key. If you want to enforce uniqueness on 2 or more columns, then we use unique key constraint.

84 What is the difference between Primary key constraint and Unique key constraint? This question is asked very frequently in interviews. 1. A table can have only one primary key, but more than one unique key 2. Primary key does not allow nulls, where as unique key allows one null

85 Drop The Unique Key To drop the constraint 1. Right click the constraint and delete. 2. Using a query Alter Table tblperson Drop Constraint UK_tbl_Person_

86 Select Statement

87 Basic select statement syntax Select Statement: is a statement used to view (select) column(s) with or without any condition. Let at first create the table called (Persons) by use the query below:

89 Let s insert the information shown to the table (Persons)

Appendix A. Using DML to Modify Data. Contents: Lesson 1: Adding Data to Tables A-3. Lesson 2: Modifying and Removing Data A-8

Appendix A. Using DML to Modify Data. Contents: Lesson 1: Adding Data to Tables A-3. Lesson 2: Modifying and Removing Data A-8 A-1 Appendix A Using DML to Modify Data Contents: Lesson 1: Adding Data to Tables A-3 Lesson 2: Modifying and Removing Data A-8 Lesson 3: Generating Numbers A-15 A-2 Using DML to Modify Data Module Overview