- C Generate Db Primary Keys
- Db Primary Aylward
- Db Primary Cowley St Laurence
- C Generate Db Primary Key Meaning
- C Generate Db Primary Key 2017
- C Generate Db Primary Key Data
Summary: in this tutorial, you will learn how to use MySQL primary key constraint to create the primary key for a table.
Introduction to MySQL primary key
A primary key is a column or a set of columns that uniquely identifies each row in the table. The primary key follows these rules:
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- A primary key must contain unique values. If the primary key consists of multiple columns, the combination of values in these columns must be unique.
- A primary key column cannot have
NULLvalues. Any attempt to insert or updateNULLto primary key columns will result in an error. Note that MySQL implicitly adds aNOT NULLconstraint to primary key columns. - A table can have one an only one primary key.
Because MySQL works faster with integers, the data type of the primary key column should be the integer e.g., INT, BIGINT. And you should ensure sure that value ranges of the integer type for the primary key are sufficient for storing all possible rows that the table may have.
A primary key column often has the AUTO_INCREMENT attribute that automatically generates a sequential integer whenever you insert a new row into the table.
When you define a primary key for a table, MySQL automatically creates an index called PRIMARY.
MySQL PRIMARY KEY examples
The PRIMARY KEY constraint allows you to define a primary key of a table when you create or alter table.
1) Define a PRIMARY KEY constraint in CREATE TABLE
Typically, you define the primary key for a table in the CREATE TABLE statement.
If the primary key has one column, you can use the PRIMARY KEY constraint as a column constraint:
When the primary key has more than one column, you must use the PRIMARY KEY constraint as a table constraint.
In this syntax, you separate columns in the column_list by commas (,).
The PRIMARY KEY table constraint can be used when the primary key has one column:
The following example creates a table named users whose primary key is the user_id column:
This statement creates the roles table that has the PRIMARY KEY constraint as the table constraint:
In case the primary key consists of multiple columns, you must specify them at the end of the CREATE TABLE statement. You put a comma-separated list of primary key columns inside parentheses followed the PRIMARY KEY keywords.
The following example creates the user_roles table whose primary key consists of two columns: user_id and role_id. It defines the PRIMARY KEY constraint as the table constraint:
Note that the statement also created two foreign key constraints.
C Generate Db Primary Keys
2) Define PRIMARY KEY constraints using ALTER TABLE
If a table, for some reasons, does not have a primary key, you can use the ALTER TABLEstatement to add a primary key to the table as follows:
The following example adds the id column to the primary key.
First, create the pkdemos table without a primary key.
Second, add a primary key to the pkdemos table using the ALTER TABLE statement:
If you add a primary key to a table that already has data. The data in the column(s), which will be included in the primary key, must be unique and not NULL.
PRIMARY KEY vs. UNIQUE KEY vs. KEY
KEY is the synonym for INDEX. You use the KEY when you want to create an index for a column or a set of columns that is not the part of a primary key or unique key.
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A UNIQUE index ensures that values in a column must be unique. Unlike the PRIMARY index, MySQL allows NULL values in the UNIQUE index. In addition, a table can have multiple UNIQUE indexes.
Suppose that email and username of users in the users table must be unique. To enforce thes rules, you can define UNIQUE indexes for the email and username columns as the following statement:
Add a UNIQUE index for the username column:
Add a UNIQUE index for the email column:
In this tutorial, you have learned how to create a primary key for a new table or add a primary key to an existing table.
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A column or columns is called primary key (PK) that uniquely identifies each row in the table.
If you want to create a primary key, you should define a PRIMARY KEY constraint when you create or modify a table.
When multiple columns are used as a primary key, it is known as composite primary key.
In designing the composite primary key, you should use as few columns as possible. It is good for storage and performance both, the more columns you use for primary key the more storage space you require.
Inn terms of performance, less data means the database can process faster.
Points to remember for primary key:
- Primary key enforces the entity integrity of the table.
- Primary key always has unique data.
- A primary key length cannot be exceeded than 900 bytes.
- A primary key cannot have null value.
- There can be no duplicate value for a primary key.
- A table can contain only one primary key constraint.
Db Primary Aylward
When we specify a primary key constraint for a table, database engine automatically creates a unique index for the primary key column.
Main advantage of primary key:
The main advantage of this uniqueness is that we get fast access.
In oracle, it is not allowed for a primary key to contain more than 32 columns.
Db Primary Cowley St Laurence
SQL primary key for one column:
The following SQL command creates a PRIMARY KEY on the 'S_Id' column when the 'students' table is created.
C Generate Db Primary Key Meaning
MySQL:
SQL Server, Oracle, MS Access:
SQL primary key for multiple columns:
C Generate Db Primary Key 2017
MySQL, SQL Server, Oracle, MS Access:
Note:you should note that in the above example there is only one PRIMARY KEY (pk_StudentID). However it is made up of two columns (S_Id and LastName).
SQL primary key on ALTER TABLE
When table is already created and you want to create a PRIMARY KEY constraint on the 'S_Id' column you should use the following SQL:
Primary key on one column:
Primary key on multiple column:
When you use ALTER TABLE statement to add a primary key, the primary key columns must not contain NULL values (when the table was first created).
C Generate Db Primary Key Data
How to DROP a PRIMARY KEY constraint?
If you want to DROP (remove) a primary key constraint, you should use following syntax:
MySQL:
SQL Server / Oracle / MS Access: