Flash cards
Review the key moves
What is the main idea behind SQL CONSTRAINT Keyword?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
___ TABLE PersonsPut the learning moves in the order that makes the concept easiest to apply.
Add Constraint
The ADD CONSTRAINT command is used to create a constraint after a table is already created.
The following SQL adds a constraint named "PK_Person" that is a PRIMARY KEY constraint on multiple columns (ID and LastName):
Example
ALTER TABLE Persons
ADD CONSTRAINT PK_Person PRIMARY KEY (ID,LastName);Drop Constraint
The DROP CONSTRAINT command is used to delete a UNIQUE, PRIMARY KEY, FOREIGN KEY, or CHECK constraint.
DROP a UNIQUE Constraint
To drop a UNIQUE constraint, use the following SQL:
SQL Server / Oracle / MS Access:
ALTER TABLE Persons
DROP CONSTRAINT UC_Person;MySQL
ALTER TABLE Persons
DROP INDEX UC_Person;DROP a PRIMARY KEY Constraint
To drop a PRIMARY KEY constraint, use the following SQL:
SQL Server / Oracle / MS Access:
ALTER TABLE Persons
DROP CONSTRAINT PK_Person;MySQL
ALTER TABLE Persons
DROP PRIMARY KEY;DROP a FOREIGN KEY Constraint
To drop a FOREIGN KEY constraint, use the following SQL:
SQL Server / Oracle / MS Access:
ALTER TABLE Orders
DROP CONSTRAINT FK_PersonOrder;MySQL
ALTER TABLE Orders
DROP FOREIGN KEY FK_PersonOrder;DROP a CHECK Constraint
To drop a CHECK constraint, use the following SQL:
SQL Server / Oracle / MS Access:
ALTER TABLE Persons
DROP CONSTRAINT CHK_PersonAge;MySQL
ALTER TABLE Persons
DROP CHECK CHK_PersonAge;