SQL-Constraints

Six types of constraints

1. Primary Key Constraint (Candidate Key)

• Does not allow the entry of Null values
• Does not allow the entry of duplicate values
• Primary Key is the column that uniquely identifies a Row in a table

2. Unique Key Constraint

• Does not allow the duplicate values
• Does allow the Null values
• Unique Key is the column that identifies a row in the table

3. Foreign Key Constraint (Integrity Constraint)

• FK is the column that is derived from the primary key column of the previous table

4. Default Constraint

• Boolean Values (True/False, M/F)

5. Not Null Constraint

• Does not allow the entry of Null values

6. Check Constraint

• A conditional statement with aggregate functions
• Where condition and Having clause together
• Having clause will be used for Aggregate functions

1. Equi Join (-) = sign 

          Two tables: (Employee and Attendance)

          (both tables have columns empno)

          Select * from Employee A, Attendance B

          where A.empno = B.Empno;

2. Inner Join (+) on the inside of the where condition before = sign

         Select * from Employee A, Attendance B

         Inner join on Employee A

         Where A. Empno(+) = B. Empno

3. Outer Join (+) on the outer part of the where condition after = sign

          Select *n  from Employee A, Attendance B

          Outer Join on Attendance B

          Where A. empno = B.empno(+);