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Can a primary key be non-unique?

Can a primary key be non-unique?

3 Answers. Primary keys are unique by deifinition; a better approach would be to have either a surrogate key (like an auto numbered value), or a composite key which covers multiple columns, such as the postal code and property name/number.

Is Oracle primary key unique?

A primary key is a column of a combination of columns in a table that uniquely identifies a row in the table. A primary key column cannot contain a NULL value or an empty string. A primary key value must be unique within the entire table.

Do we need unique index on primary key?

When you create a PRIMARY KEY constraint, a unique clustered index on the column or columns is automatically created if a clustered index on the table does not already exist and you do not specify a unique nonclustered index. The primary key column cannot allow NULL values.

What is non-unique index in Oracle?

Creating a Unique Index Explicitly Indexes can be unique or non-unique. Unique indexes guarantee that no two rows of a table have duplicate values in the key column (or columns). Non-unique indexes do not impose this restriction on the column values. Use the CREATE UNIQUE INDEX statement to create a unique index.

Can a primary key be a varchar?

It is perfectly acceptable to use a varchar column as the primary key.

Why do we use unique keys?

The purpose of a unique key is to make sure that information in the column for each table record is unique. When you allow the user to enter the null value. Unique key is used because it creates a non-clustered index by default. Unique key can be used when you have to keep null values in column.

What is difference between primary and unique key?

A primary key is a column of table which uniquely identifies each tuple (row) in that table. Unique key constraints also identifies an individual tuple uniquely in a relation or table. A table can have more than one unique key unlike primary key.

Why must a primary key be unique?

primary key :- It should be used when you have to give uniquely identify a row. primary is key which unique for every row in a database constraint is that it doesn’t allow null in it.so, you might have seen that the database have a column which is auto increment and it is the primary key of the table.

Is composite key unique?

A composite unique key is a unique key made up of a combination of columns. Oracle creates an index on the columns of a unique key, so a composite unique key can contain a maximum of 16 columns. Any row that contains nulls in all key columns automatically satisfies the constraint.

What is the unique index?

Unique indexes are indexes that help maintain data integrity by ensuring that no two rows of data in a table have identical key values. When you create a unique index for an existing table with data, values in the columns or expressions that comprise the index key are checked for uniqueness.

Do indexes need to be unique?

A PRIMARY index is intended as a primary means to uniquely identify any row in the table, so unlike UNIQUE it should not be used on any columns which allow NULL values. Your PRIMARY index should be on the smallest number of columns that are sufficient to uniquely identify a row.

What’s the difference between unique index and primary key index?

Otherwise a primary key will happen to create a unique index. Hence, there is no real difference here. Under the covers there will be either a unique or non-unique index.

How to create a foreign key to a non primary key?

If you really want to create a foreign key to a non-primary key, it MUST be a column that has a unique constraint on it. From Books Online: A FOREIGN KEY constraint does not have to be linked only to a PRIMARY KEY constraint in another table; it can also be defined to reference the columns of a UNIQUE constraint in another table.

Which is better a NOT NULL or a primary key?

The benefit of a primary key over a NOT NULL and UNIQUE index is mostly semantics. A primary key has meaning — it conveys something about the data. A not null constraint plus a unique index does not convey that same information.

How to create a table with no primary key?

Let’s fix that: SQL> 1 1* CREATE TABLE USER SQL> a s 1* CREATE TABLE USERs SQL> l 1 CREATE TABLE USERs 2 ( 3 ID NUMBER NOT NULL , 4 CONSTRAINT U_PK PRIMARY KEY ( ID ) ENABLE 5* ) SQL> r 1 CREATE TABLE USERs 2 ( 3 ID NUMBER NOT NULL , 4 CONSTRAINT U_PK PRIMARY KEY ( ID ) ENABLE 5* ) Table created.