; Use ALTER TABLE command to add the needed FOREIGN KEY‘s back to the table. I have a table in PostgreSQL where the schema looks like this: CREATE TABLE "foo_table" ( "id" serial NOT NULL PRIMARY KEY, "permalink" varchar(200) NOT NULL, "text" varchar(512) NOT NULL, "timestamp" timestamp with time zone NOT NULL ) Now I want to make the permalink unique across the table by ALTER-ing the table. Example. PostgreSQL – Make an existing column as PRIMARY KEY. The tool then generates the appropriate alter table SQL command for adding the constraint to the table. (Note that the CONSTRAINT keyword must be omitted.) ALTER TABLE distributors DROP CONSTRAINT zipchk , ADD CONSTRAINT zipchk CHECK (length(zipcode) = 6); ALTER CONSTRAINT in Postgres 9.4 or later (like you found) can only change the "deferability" of a FK constraints. – jpmc26 Nov 26 '14 at 1:41 6 As of version 9.4, PostgreSQL supports ALTER TABLE ... ALTER CONSTRAINT for foreign keys. For example, the following PostgreSQL statement creates a new table called COMPANY5 and adds five columns. Current Structure. Well, this not directly altering FOREIGN KEY constraint, and there are DROP and ADD still, though this is only one statement: ALTER table chemlab.rule_header DROP CONSTRAINT rule_header_parent_id_fkey, ADD CONSTRAINT rule_header_parent_id_fkey FOREIGN KEY (parent_id) REFERENCES chemlab.rule_header(id) ON DELETE RESTRICT; This features will "Allow constraint attributes to be altered, so the default setting of NOT DEFERRABLE can be altered to DEFERRABLE and back." Make a Column as PRIMARY KEY. PostgreSQL 11.2 add constraints, delete constraints, add columns, delete columns. The name (possibly schema-qualified) of an existing table to alter. Which table needs to be operated explicitly * can be appended to the table name to indicate that descendant tables are to be scanned, but in the current version, this is the default behavior. To change the structure of an existing table, you use PostgreSQL ALTER TABLE statement.. Photo by Richard Payette on Unsplash Steps. Delete constraint [syntax general] alter table table_name drop constraint “some_name”; Notes: 1. If ONLY is specified, only that table is altered. We will follow this order to update the FOREIGN KEY‘s.. Use ALTER TABLE command to drop any existing FOREIGN KEY‘s. This might help, although it may be a bit of a dirty hack: create or replace function create_constraint_if_not_exists ( t_name text, c_name text, constraint_sql text ) returns void AS $$ begin -- Look for our constraint if not exists (select constraint_name from information_schema.constraint_column_usage where table_name = t_name and constraint_name = c_name) then execute constraint… If the condition evaluates to false, the record violates the constraint and is not entered into the table. table. If ONLY is not specified, the table and all its descendant tables (if any) are updated. The RazorSQL alter table tool includes an Add Constraint option for adding check constraints to PostgreSQL database tables. So not what you are looking for. With the below table structure, we can see three FOREIGN KEY constraints. No column in this table is marked PRIMARY KEY. Summary: in this tutorial, you will learn how to use the PostgreSQL ALTER TABLE statement to modify the structure of a table.. Introduction to PostgreSQL ALTER TABLE statement. Consider the following table named students. Modify the table. You can make an existing column of PostgreSQL Table as PRIMARY KEY using ALTER TABLE query and adding a constraint. Here, we add a CHECK with SALARY column, so … The add constraint function allows the user to add a constraint name and a constraint condition. Now, we will make the column id as PRIMARY KEY. 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