SQL INTERVIEW QUESTION- Difference Between Primary Key & Unique Key

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This is the third video of SQL INTERVIEW QUESTIONS Playlist, and in this video we are discussing the second interview question i.e What is the difference between Primary key & Unique Key

SQL constraints are used to specify rules for data in a table.
Constraints can be specified when the table is created with the CREATE TABLE statement, or after the table is created with the ALTER TABLE statement.

SQL constraints are used to specify rules for the data in a table.
Constraints are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the table. If there is any violation between the constraint and the data action, the action is aborted.

Constraints can be column level or table level. Column level constraints apply to a column, and table level constraints apply to the whole table.

The following constraints are commonly used in SQL:

NOT NULL - Ensures that a column cannot have a NULL value
UNIQUE - Ensures that all values in a column are different
PRIMARY KEY - A combination of a NOT NULL and UNIQUE. Uniquely identifies each row in a table
FOREIGN KEY - Prevents actions that would destroy links between tables
CHECK - Ensures that the values in a column satisfies a specific condition
DEFAULT - Sets a default value for a column if no value is specified
CREATE INDEX - Used to create and retrieve data from the database very quickly

The UNIQUE constraint ensures that all values in a column are different.
Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns.
A PRIMARY KEY constraint automatically has a UNIQUE constraint.
However, you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.

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).

Testing Null Values: https://www.youtube.com/watch?v=2OAuGTekyD0&t=7s
SQL Constraints: https://www.youtube.com/watch?v=e0MpCcvUazQ&t=6s
SQL Constraints Part 2: https://www.youtube.com/watch?v=AE8MmTUsQ1o&t=82s

A primary key, also called a primary keyword, is a key in a relational database that is unique for each record. It is a unique identifier, such as a driver license number, telephone number (including area code), or vehicle identification number (VIN). A relational database must always have one and only one primary key.

A column or columns is called primary key (PK) that uniquely identifies each row in the table.
If you want to create a primary key, you should define a PRIMARY KEY constraint when you create or modify a table.
When multiple columns are used as a primary key, it is known as composite primary key.
In designing the composite primary key, you should use as few columns as possible. It is good for storage and performance both, the more columns you use for primary key the more storage space you require.

SQL is a very important topic if you want to pursue your career in Data Science/Analytics. It helps you pull the data from the different databases and let you perform the analysis.
If you are an aspiring Data Scientist/Analyst , do follow this SQL playlist from the start.

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