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SQL Installing SSMS
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SQL Syntax
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SQL Datatype
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SQL Database
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SQL Table
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SQL Select
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SQL Distinct
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SQL Count
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SQL Top
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SQL Aliases
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SQL In
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SQL Insert
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SQL Update
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SQL Null
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SQL Like
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SQL Wildcards
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SQL Nested Query
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SQL Between
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SQL Unique
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SQL Primary Key
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SQL Foreign Key
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SQL Check
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SQL Default
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SQL Database Diagram
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SQL Stored Procedures
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SQL Joins
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SQL Inner Join
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SQL Left Join
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SQL Right Join
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SQL Full Join
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SQL Trigger
SQL Primary Key
The PRIMARY KEY constraint uniquely identifies each record in a table. Primary keys must contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).
Example:
CREATE TABLE TableName (ColumnName1 datatype PRIMARY KEY, ColumnName2 datatype,…., ColumnNameN datatype);
The following SQL creates a PRIMARY KEY on the “ID” column when the “Persons” table is created:
CREATE TABLE Persons (
ID int PRIMARY KEY,
LastName varchar(255) ,
FirstName varchar(255),
Age int
);
To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:
CREATE TABLE Persons (
ID int,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
CONSTRAINT PK_Person PRIMARY KEY (ID,LastName)
);
Note: In the example above there is only ONE PRIMARY KEY (PK_Person). However, the VALUE of the primary key is made up of TWO COLUMNS (ID + LastName).
To create a PRIMARY KEY constraint on the “ID” column when the table is already created, use the following SQL:
ALTER TABLE Persons ADD PRIMARY KEY (ID);
To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:
ALTER TABLE Persons ADD CONSTRAINT PK_Person PRIMARY KEY (ID,LastName);
Note: If you use ALTER TABLE to add a primary key, the primary key column(s) must have been declared to not contain NULL values (when the table was first created).
To drop a PRIMARY KEY constraint, use the following SQL:
ALTER TABLE Persons DROP CONSTRAINT PK_Person;
Course Video
1. Creates a PRIMARY KEY on the “ID” column when the table is created and also add not null constrain
2.Create table with 5 column and add primary key to the first column
Hint: Add primary key to the employee id column
3.Create a table without primary key and add primary key using alter table
Hint: Primary key was added to the customer id column by alter table
4.create a new table with sales name and add the primary key with the combination of the two column
Hint: Primary key was added to the sales id and product id
5. Write a sql query to drop primary key constrain
YouTube Reference :
1) SQL Primary Key in Hindi/Urdu
2) SQL Primary Key in English
