Can A Primary Key Be Generated From Several Existing Field

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The primary key should be the first field (or fields) in your table design. While most databases allow you to define primary keys on any field in your table, the common convention and what the next developer will expect, is the primary key field(s) coming first. Use a Meaningless Primary Key. The values in a table's primary key should never change over time. Jul 18, 2019  Technically Yes you can do that provided that it’s a composite primary key and it actually makes sense in some cases.

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SQL – PRIMARY KEY:

  • PRIMARY KEY is a constraint in SQL which is used to identify each record uniquely in a table.
  • By default, PRIMARY KEY is UNIQUE.
  • PRIMARY KEY can’t have null values.
  • 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.
  • Syntax for SQL PRIMARY KEY & COMPOSITE KEY constraint is given below.

SQL Syntax for PRIMARY KEY constraint:

SQL Syntax for PRIMARY KEY constraintCREATE TABLE table_name
(
column_name1 data_type(size) PRIMARY KEY,
column_name2 data_type(size) NOT NULL,
column_name3 data_type(size),
etc…
);OrCREATE TABLE table_name
(
column_name1 data_type(size) NOT NULL,
column_name2 data_type(size) NOT NULL,
column_name3 data_type(size),
etc…
PRIMARY KEY column_name1;
);
SQL Syntax for COMPOSITE KEY constraintCREATE TABLE table_name
(
column_name1 data_type(size) NOT NULL,
column_name2 data_type(size) NOT NULL,
column_name3 data_type(size),
CONSTRAINT Constraint_name PRIMARY KEY (column_name1, column_name2)
etc…
);

Example1 for how to use SQL PRIMARY KEY constraint:

  • Please execute below query in SQL to create a table with PRIMARY KEY constraint.
CREATE TABLE student
(Student_ID int PRIMARY KEY,
Student_name varchar(255) NOT NULL,
City varchar(255),
Marks int);

Can A Primary Key Be Generated From Several Existing Field In One

  • Now, a new table called “student” is created and the fields “Student_ID” is defined as PRIMARY KEY. So, this column will be NOT NULL & UNIQUE.

Example2 for how to use SQL PRIMARY KEY constraint on multiple columns (COMPOSITE KEY):

  • Please execute below query in SQL to create a table with COMPOSITE KEY i.e. PRIMARY KEY constraint on multiple columns.
CREATE TABLE student
(Student_ID int NOT NULL,
Student_name varchar(255) NOT NULL,
City varchar(255),
Marks int
CONSTRAINT CompKey_ID_NAME PRIMARY KEY (Student_ID, Student_name));
  • Now, a new table called “student” is created and the fields “Student_ID” & “Student_name” is together defined as PRIMARY KEY.
  • CompKey_ID_NAME is the Constraint name. So, the combination of Student_ID and Student_name will be NOT NULL & UNIQUE.

How to ALTER a PRIMARY KEYconstraint?

Can A Primary Key Be Generated From Several Existing Fields

  • To ALTER a PRIMARY KEY constraint on an existing table, you can use below Syntax.

Syntax for PRIMARY KEY:


ALTER table table_name
ADD PRIMARY KEY column_name;

Example for PRIMARY KEY:

ALTER table Student
ALTER PRIMARY KEY Marks;
  • To ALTER a COMPOSITE KEY constraint on an existing table, you can use below Syntax.

Syntax for COMPOSITE KEY:

ALTER table table_name
ADD CONSTRAINT Constraint_name PRIMARY KEY (column_name1, column_name2);

Example for COMPOSITE KEY:

Can A Primary Key Be Generated From Several Existing Field In Outlook

ALTER table Student
ADD CONSTRAINT CompKey_CityMarks PRIMARY KEY ( City, Marks );

How to DROP a PRIMARY KEY & COMPOSITE KEY constraint?

  • To DROP a PRIMARY KEY constraint on an existing table, you can use below Syntax.

Syntax:Generate private key with password.

ALTER table table_name
DROP PRIMARY KEY;

Can A Primary Key Be Generated From Several Existing Field In Word

Example:

ALTER table Student
DROP PRIMARY KEY;
  • To DROP a COMPOSITE KEY constraint on an existing table, you can use below Syntax.

Syntax:

ALTER table table_name
DROP CONSTRAINT Constraint_name;

Example:

ALTER table Student
DROP CONSTRAINT CompKey_CityMarks;

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