Quick Answer: Can A Table Have Two Primary Keys?

Does a foreign key have to be a primary key?

A foreign key must refer to an entire primary key, and not just part of it.

Consider a Department table with a primary key of company_name + department_name.

An Employee table should only refer to both attributes and not to department_name alone.

Lack of foreign key indexes..

Can we delete a table with foreign key?

You can alter a foreign key constraint with delete cascade option as shown below. This will delete chind table rows related to master table rows when deleted. If you have multiply rows to delete and you don’t want to alter the structure of your tables you can use cursor.

Should Join table have primary key?

A table must have exactly one primary key to qualify as relational, but that key can be composed of multiple columns. A foreign key, by contrast, is one or more fields or columns that corresponds to the primary key of another table. Foreign keys are what make it possible to join tables to each other.

Can a table only have a foreign key?

1 Answer. There is no problem having a table that consists of foreign keys only. … If you add further columns in the future there will be no problems due to the existing columns all being foreign keys. This situation is very common when there is a many-to-many relationship between entity types.

Does primary key allow null values?

Primary key constraints A primary key defines the set of columns that uniquely identifies rows in a table. When you create a primary key constraint, none of the columns included in the primary key can have NULL constraints; that is, they must not permit NULL values. … NULL values are not allowed.

What is primary key and example?

A primary key is either an existing table column or a column that is specifically generated by the database according to a defined sequence. For example, students are routinely assigned unique identification (ID) numbers, and all adults receive government-assigned and uniquely-identifiable Social Security numbers.

How do I create multiple primary keys in a table?

A Table can have a Composite Primary Key which is a primary key made from two or more columns. For example: CREATE TABLE userdata ( userid INT, userdataid INT, info char(200), primary key (userid, userdataid) );

What happens if we remove primary key from SQL table?

You can delete (drop) a primary key in SQL Server by using SQL Server Management Studio or Transact-SQL. When the primary key is deleted, the corresponding index is deleted.

Can we drop primary key constraint?

You cannot drop a primary key constraint if another table has a foreign key constraint that references the primary key. To drop the primary key, you must first drop all foreign keys that reference it.

Can we use 2 primary keys in a table?

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 table have 3 primary keys?

A table can only ever have a one primary key. … You can create a table with two different unique indexes (which are much like a primary key) but only one primary key can exist. What you’re asking about is whether you should have a composite or compound primary key; a primary key using more than one column.

Can a table have no primary key?

Every table can have (but does not have to have) a primary key. The column or columns defined as the primary key ensure uniqueness in the table; no two rows can have the same key.

What is the difference between primary key and unique key?

Primary Key is a column that is used to uniquely identify each tuple of the table. It is used to add integrity constraints to the table. Only one primary key is allowed to be used in a table. … Unique key is a constraint that is used to uniquely identify a tuple in a table.

How many unique keys can a table have?

A table can have more than one unique key unlike primary key. Unique key constraints can accept only one NULL value for column. Unique constraints are also referenced by the foreign key of another table.

Can we join two tables without primary key?

The longer answer is yes, there are a few ways to combine two tables without a common column, including CROSS JOIN (Cartesian product) and UNION. The latter is technically not a join but can be handy for merging tables in SQL. In this article, I’ll guide you through the different solutions with examples.

Can Unique Key be a primary key?

A primary key must be unique. A unique key does not have to be the primary key – see candidate key. That is, there may be more than one combination of columns on a table that can uniquely identify a row – only one of these can be selected as the primary key.

How do I make multiple primary keys in SQL?

A table can have only one PRIMARY KEY either on one column or multiple columns. When multiple columns are defined as PRIMARY KEY, then, it is called COMPOSITE KEY. If we try to insert/update duplicate values for the PRIMARY KEY column, then, the query will be aborted.

How can I have two primary keys?

Select the field or fields that you want to use as the primary key. To select one field, click the row selector for the field you want. To select more than one field to create a composite key, hold down CTRL and then click the row selector for each field. On the Design tab, in the Tools group, click Primary Key.

Can you have duplicate primary keys?

Since both primary key and unique columns do not accept duplicate values, they can be used for uniquely identifying a record in the table. This means that, for each value in the primary or unique key column, only one record will be returned.

Can a table have multiple alternate keys?

Yes, MySQL allows this. You can have multiple foreign keys on the same table. The foreign keys in your schema (on Account_Name and Account_Type ) do not require any special treatment or syntax. … It appears that at least one of these cases applies to the ID and Name columns in the Customer table.

How do you check if a column is a primary key in SQL?

At the select statement, add CONSTRAINT_NAME column from INFORMATION_SCHEMA. KEY_COLUMN_USAGE that will give you null or keyname. this query return column with is primary key.