43. What is PRIMARY KEY?
A PRIMARY KEY constraint is a unique identifier for a row within a database table. Every table should have a primary key constraint to uniquely identify each row and only one primary key constraint can be created for each table. The primary key constraints are used to enforce entity integrity.
44. What is UNIQUE KEY constraint?
A UNIQUE constraint enforces the uniqueness of the values in a set of columns, so no duplicate values are entered. The unique key constraints are used to enforce entity integrity as the primary key constraints.
45. What is FOREIGN KEY?
A FOREIGN KEY constraint prevents any actions that would destroy links between tables with the corresponding data values. A foreign key in one table points to a primary key in another table. Foreign keys prevent actions that would leave rows with foreign key values when there are no primary keys with that value. The foreign key constraints are used to enforce referential integrity.
46. What is CHECK Constraint?
A CHECK constraint is used to limit the values that can be placed in a column. The check constraints are used to enforce domain integrity.
47. What is NOT NULL Constraint?
A NOT NULL constraint enforces that the column will not accept null values. The not null constraints are used to enforce domain integrity, as the check constraints.
48. How to get @@ERROR and @@ROWCOUNT at the same time?
If @@Rowcount is checked after Error checking statement then it will have 0 as the value of @@Recordcount as it would have been reset. And if @@Recordcount is checked before the error-checking statement then @@Error would get reset. To get @@error and @@rowcount at the same time do both in same statement and store them in local variable.
SELECT @RC = @@ROWCOUNT, @ER = @@ERROR
SQL Server Common Questions and Answers