CONSTRAINTS are used to check validations before inserting data into Table and also provide relationship between tables.
TABLE LEVEL CONSTRAINTS : 1. UNIQUE CONSTRAINT
2. PRIMARY KEY CONSTRAINT
3. CHECK CONSTRAINT
4. FOREIGN KEY CONSTRAINT
COLUMNLEVEL CONSTRAINTS : 1. UNIQUE CONSTRAINT
2. PRIMARY KEY CONSTRAINT
3. CHECK CONSTRAINT
4. FOREIGN KEY CONSTRAINT
5. NOT NULL CONSTRAINT
6. DEFAULT CONSTRAINT.
NOT NULL Constraint - Disallows NULL values in a table's column.
UNIQUE Constraint - Disallows duplicate values in a column or set of columns.
PRIMARY KEY Constraint - Disallows duplicate values and NULL values in a column or set
of columns.
FOREIGN KEY Constraint - Require each value in a column or set of columns match a
value in a related table's UNIQUE or PRIMARY KEY.
CHECK Constraint - Disallows values that do not satisfy the logical expression of
the constraint.
