![]() State capitals also have an extra column, state, that shows their state. In this case, the capitals table inherits all the columns of its parent table, cities. We define the capitals table so that it inherits from cities: However, what happens when we want to ask for data about a city, regardless of whether it is a capital or not? The inheritance feature can help to resolve this problem. This can be done by creating two tables, one for state capitals and one for cities that are not capitals. We want to be able to quickly retrieve the capital city for any particular state. Each state has many cities, but only one capital. Let's start with an example: suppose we are trying to build a data model for cities. (SQL:1999 and later define a type inheritance feature, which differs in many respects from the features described here.) This blog post has explained how to add or drop a primary key constraint in Postgres via practical examples.PostgreSQL implements table inheritance, which can be a useful tool for database designers. For dropping a primary key constraint, the DROP CONSTRAINT is used with the ALTER TABLE command. You can add the primary key to an existing table using the Postgres ALTER TABLE command. Tables can be created with a primary key constraint using the CREATE TABLE statement. This way, a primary key constraint can be added or deleted from a table in Postgres. The output clarifies that the primary key constraint has been removed successfully. Let’s verify the constraint deletion via the below command: SELECT * FROM staff_bio To drop a primary key constraint, use the ALTER TABLE command with DROP CONSTRAINT as follows: ALTER TABLE staff_bio The output shows that a primary key has been added to an existing table: st_id column. Let’s validate the table’s structure via the following command: SELECT * FROM staff_bio The “ALTER TABLE” message in the output window proves that the “staff_bio” table has been modified successfully. ADD CONSTRAINT adds a primary key constraint in Postgres, such as “st_id_pk”. ALTER TABLE is a command used to modify the staff_bio table. Let’s run the ALTER TABLE command to add a PRIMARY KEY constraint in the staff_bio table: ALTER TABLE staff_bio The output snippet proves that the “staff_bio” table has no primary key constraint. For better understanding, firstly, we will create a table without any constraint: CREATE TABLE staff_bio(Įxecuting the SELECT command will show you the structure of the staff_bio table: SELECT * FROM staff_bio ![]() ![]() "ALTER TABLE" lets you add primary key constraints to existing Postgres tables. You can verify the table’s creation via the below command: SELECT * FROM staff_information įrom the table’s structure, you can clearly observe that a primary key constraint has been added to the staff_information table.Īdding PRIMARY KEY Using ALTER TABLE Command The “ CREATE TABLE” message in the output window indicates that the “staff_information” table has been created. St_id INT CONSTRAINT st_id_pk PRIMARY KEY, We will create a sample table with the following columns: st_id, st_name, st_department, and st_age: CREATE TABLE staff_information( To do so, we will add a primary key constraint in the staff_information table using the CREATE TABLE command. Let’s add a primary constraint during table creation. Adding PRIMARY KEY While Table Creation - Adding PRIMARY KEY Using ALTER TABLE Command - Dropping PRIMARY KEY CONSTRAINT This Postgres blog will cover the below-listed aspects of the Primary key constraint: In Postgres, tables can be created with a primary key constraint using the CREATE TABLE command or altered using the ALTER TABLE command. Users can add/set a primary key at the time of table creation or to an existing table. In Postgres, Primary keys are used to uniquely identify a table’s record.
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