Oracle database Add, query, delete primary KEY constraints

A PRIMARY KEY constraint has only one in each table, but a PRIMARY key constraint can consist of multiple columns

You can create a PRIMARY KEY constraint when you create a table, or you can add a primary key constraint by using the ALTER statement

1: Add a PRIMARY KEY constraint

"1"

Syntax :

CREATE Table table name (field name primary key, field name ...);

Example:

CREATE TABLE Student (ID number primary key,name varchar), age number (3), email varchar2 (50),);

"2"

Syntax :

CREATE Table table name (field name, field name ...), [CONSTRAINT constraint name] primary key (field name, field name)--one column only needs to write one, multiple columns specify primary key at the same time, separated by ', ' number;

Example:

CREATE TABLE Student (ID number, name varchar), age number (3), email varchar2, primary key (Id,name )--one column only needs to write one, multiple columns simultaneously specify the primary key, separated by the ', ' number;

"3"

Syntax :

ALTER table name ADD CONSTRAINT constraint name PRIMARY KEY (field name);

Example:

ALTER TABLE STUDENT ADD CONSTRAINT pk_id PRIMARY KEY (ID);

2: Querying PRIMARY KEY constraints

Syntax:

Select cu.* from User_cons_columns cu, user_constraints au where cu.constraint_name = Au.constraint_name and Au.constrai Nt_type = ' P ' and au.table_name = ' table to query ';

Example:

Select cu.* from User_cons_columns cu, user_constraints au where cu.constraint_name = Au.constraint_name and Au.constraint _type = ' P ' and au.table_name = ' STUDENT ';

3: Delete primary KEY constraint

Grammar:

ALTER TABLE table_name DROP CONSTRAINT constraint_name;

Example:

To add a primary key when creating a table, first query out the primary key name, then delete

ALTER TABLE STUDENT DROP CONSTRAINT pk_stu_id;

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Oracle database Add, query, delete primary KEY constraints