Primary key for multiple column in PostgreSQL?

问题:

How to provide primary key for multiple column in a single table using PostgreSQL?

Example:

Create table "Test" 
(
   "SlNo" int not null primary key,
   "EmpID" int not null, /* Want to become primary key */
   "Empname" varchar(50) null,
   "EmpAddress" varchar(50) null
);

Note: I want to make "EmpID" also a primary key.

回答1:

There can only be one primary key per table - as indicated by the word "primary".

You can have additional UNIQUE columns like:

CREATE TABLE test(
   sl_no int PRIMARY KEY,  -- NOT NULL automatically
   emp_id int UNIQUE NOT NULL,
   emp_name text,
   emp_addr text
);

Or to make that a single multicolumn primary key, use a table constraint instead of a column constraint:

CREATE TABLE test(
   sl_no int,     -- NOT NULL automatically
   emp_id int ,   -- NOT NULL automatically
   emp_name text,
   emp_addr text,
   PRIMARY KEY (sl_no, emp_id)
);

(Multicolumn UNIQUE constraints are possible, too.)

Aside: Don't use CaMeL-case identifiers in Postgres. Use legal, lower-case identifiers so you never have to use double-quotes. Makes your life easier.