Add Constraint SQL - sql - sql tutorial - learn sql
- SQL Constraints are rules, which is used to limit the type of data that can go into a table.
- To maintain the accuracy & integrity of the data within the table.
- Constraints are used to make sure that the integrity of data is maintained in the database.
- Constraints can be divided into two types as follows,
Column level constraints :
- limits only column data
Table level constraints :
- limits whole table data
- Following are the most used constraints that can be applied to a table.
- Not null
- Unique
- Primary key
- Foreign key
- Check
- Default
Tags : sql tutorial , pl sql tutorial , mysql tutorial , oracle tutorial , learn sql , sql server tutorialSQL > ALTER TABLE > Add Constraint Syntax
SQL > ALTER TABLE > Add Constraint Syntax
- Sometimes we may decide to add a new constraint to an existing table (to see what are the different types of constraints that can be located on a database table, ref. to the CONSTRAINT section).
- The syntax for adding a constraint in SQL is,
ALTER TABLE "table_name"
ADD [CONSTRAINT_NAME]
[CONSTRAINT_TYPE][CONSTRAINT_CONDITION];- For example, assuming our initial point is the Employee table created in the CREATE TABLE section:
Table Employee
| Column Name | Data Type |
|---|---|
| First_Name | char(50) |
| Last_Name | char(50) |
| Address | char(50) |
| City | char(50) |
| Country | char(25) |
| Birth_Date | datetime |
- Assume we want to add a UNIQUE constraint to the "Address" column. To do this, we type in the following:
MySQL:
ALTER TABLE Employee ADD CONSTRAINT Wikitechy UNIQUE (Address);Oracle:
ALTER TABLE Employee ADD CONSTRAINT Wikitechy UNIQUE (Address);SQL Server:
ALTER TABLE Employee ADD CONSTRAINT Wikitechy UNIQUE (Address);
- where Wikitechy is the name of the constraint.
Alter Table Add Constraint
