Can I define multiple unique constraints on a table?
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.
Can we create multiple unique keys in a table?
There can be multiple unique keys defined on a table. … One or more columns make up a unique key. Column may be NULL, but on one NULL per column is allowed. A unique constraint can be referenced by a Foreign Key Constraint.
Can I define multiple unique constraints on a table in SQL?
Introduction to SQL UNIQUE constraint
A table can have only one PRIMARY KEY constraint, however, it can have multiple UNIQUE constraints. Unlike the PRIMARY KEY constraint, you can enter a NULL value for a column that has a UNIQUE constraint.
Can a table have more than one unique index?
Unlike the PRIMARY KEY index, you can have more than one UNIQUE index per table. Another way to enforce the uniqueness of value in one or more columns is to use the UNIQUE constraint. When you create a UNIQUE constraint, MySQL creates a UNIQUE index behind the scenes.
Which constraint can be enforced per table?
Explanation: Each table is having only one primary key constraint and it contains only unique values.
Can a table have 2 primary keys?
A primary key is a field or set of fields with values that are unique throughout a table. Values of the key can be used to refer to entire records, because each record has a different value for the key. Each table can only have one primary key.
Can 2 tables have same primary key?
Yes. You can have same column name as primary key in multiple tables. Column names should be unique within a table. A table can have only one primary key, as it defines the Entity integrity.
What is unique key example?
Unique key is a constraint that is used to uniquely identify a tuple in a table. Multiple unique keys can present in a table. NULL values are allowed in case of a unique key.
Difference between Primary Key and Unique Key.
|Primary Key||Unique Key|
|present in a table||present in a table|
What is the function of the unique constraint unique?
The UNIQUE constraint ensures that all values in a column are different. Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns. A PRIMARY KEY constraint automatically has a UNIQUE constraint.
Can foreign key be NULL?
Short answer: Yes, it can be NULL or duplicate. I want to explain why a foreign key might need to be null or might need to be unique or not unique. First remember a Foreign key simply requires that the value in that field must exist first in a different table (the parent table). That is all an FK is by definition.