- What is primary key mysql?
- Can a foreign key be updated?
- How do you drop a foreign key?
- Is foreign key a primary key?
- Can we drop a table with primary key?
- Is unique not null is same as primary key?
- Does foreign key allow duplicate values?
- What is primary key and foreign key?
- Why are foreign key constraints used?
- Can we delete a foreign key?
- Is a foreign key necessary?
- What is alternate key?
What is primary key mysql?
The PRIMARY KEY constraint uniquely identifies each record in a table.
Primary keys must contain UNIQUE values, and cannot contain NULL values.
A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields)..
Can a foreign key be updated?
The foreign key relation can be created either through SSMS GUI or T-SQL. Rules for update/delete operations may be specified explicitly. However if nothing is specified then the default rule is No Action. The rule may be changed to any other option at any time later by recreating the FK relation.
How do you drop a foreign key?
Dropping Foreign Key Constraints You can drop a foreign key constraint using the following ALTER TABLE syntax: ALTER TABLE tbl_name DROP FOREIGN KEY fk_symbol; If the FOREIGN KEY clause defined a CONSTRAINT name when you created the constraint, you can refer to that name to drop the foreign key constraint.
Is foreign key a primary key?
Primary key uniquely identify a record in the table. Foreign key is a field in the table that is primary key in another table. … We can have only one Primary key in a table. We can have more than one foreign key in a table.
Can we drop a table with primary key?
The syntax to drop a primary key in SQL is: ALTER TABLE table_name DROP PRIMARY KEY; table_name. The name of the table to modify.
Is unique not null is same as primary key?
PRIMARY KEY constraint differs from the UNIQUE constraint in that; you can create multiple UNIQUE constraints in a table, with the ability to define only one SQL PRIMARY KEY per each table. Another difference is that the UNIQUE constraint allows for one NULL value, but the PRIMARY KEY does not allow NULL values.
Does foreign key allow duplicate values?
Unlike primary keys, foreign keys can contain duplicate values. Also, it is OK for them to contain NULL values. Indexes aren’t automatically created for foreign keys; however, as a DBA, you can define them. A table is allowed to contain more than one foreign key.
What is primary key and foreign key?
A foreign key is a column or a set of columns in one table that references the primary key columns in another table. The primary key is defined as a column (or set of columns) where each value is unique and identifies a single row of the table.
Why are foreign key constraints used?
The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables. The FOREIGN KEY constraint also prevents invalid data from being inserted into the foreign key column, because it has to be one of the values contained in the table it points to.
Can we delete a foreign key?
Yes, you can. But you need to delete the foreign key constraint first in the database. Without the relationship constraints, you can pretty much drop the entire table containing the primary key as there are no more dependencies.
Is a foreign key necessary?
Foreign keys help enforce referential integrity at the data level. They also improve performance because they’re normally indexed by default. Foreign keys can also help the programmer write less code using things like ON DELETE CASCADE .
What is alternate key?
Alternate Key or Secondary Key is the key that has not been selected to be the primary key, but are candidate keys. However, it is considered a candidate key for the primary key. A candidate key not selected as a primary key is called alternate or secondary key.