3. SQL Constraints | Constraints in SQL | What is Unique Constraints | primary key in sql

SQL Constraints | Constraints in SQL | What is Unique Constraints | primary key in sql

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What is constraints in SQL? 00:01
Constraints are the rules enforced on the data columns of a table.​
These 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 database.​
Constraints can be specified when a table is created with the CREATE TABLE statement or you can use the ALTER TABLE statement to create constraints even after the table is created.​

Types of Constraints in SQL:​ 00:40
1. Not Null​
2. Default​
3. Unique​
4. Primary Key​
5. Foreign Key​
6. Check​
7. Index

1. NOT NULL Constraints​ 01:02
By default, a column can hold NULL values.
If you do not want a column to have a NULL value, then you need to define such a constraint on this column specifying that NULL is now not allowed for that column.​

2. DEFAULT Constraints​ 02:00
The DEFAULT constraint provides a default value to a column when the INSERT INTO statement does not provide a specific value.​

3. UNIQUE Constraints​ 02:55
The UNIQUE Constraint prevents two records from having identical values in a column. In the CUSTOMERS table.​
for example, you might want to prevent two or more people from having an identical age.​

4. PRIMARY KEY Constraints​ 04:23
A primary key is a field in a table which uniquely identifies each row/record in a database table. Primary keys must contain unique values.​
A primary key column cannot have NULL values.​
A table can have only one primary key, which may consist of single or multiple fields. When multiple fields are used as a primary key, they are called a composite key.​

5. FOREIGN KEY Constraints​ 05:40
A foreign key is a key used to link two tables together. This is sometimes also called as a referencing key.​
A Foreign Key is a column or a combination of columns whose values match a Primary Key in a different table.​

6. CHECK Constraints​ 06:50
The CHECK Constraint enables a condition to check the value being entered into a record.​
If the condition evaluates to false, the record violates the constraint and isn't entered the table.​

7. INDEX Constraints​ 07:58
The INDEX is used to create and retrieve data from the database very quickly.​
An Index can be created by using a single or group of columns in a table.​
When the index is created, it is assigned a ROWID for each row before it sorts out the data.​
Proper indexes are good for performance in large databases, but you need to be careful while creating an index.​
A Selection of fields depends on what you are using in your SQL queries.​

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