Prevent empty strings in CHARACTER VARYING field - postgresql

Question

I am using PostgreSQL and would like to prevent certain required CHARACTER VARYING (VARCHAR) fields from allowing empty string inputs.
These fields would also need to contain unique values, so I am already using a unique constraint; however, this does not prevent an original (unique) empty value.
Basic example, where username needs to be unique and not empty
| id | username | password |
+----+----------+----------+
| 1 | User1 | pw1 | #Allowed
| 2 | User2 | pw1 | #Allowed
| 3 | User2 | pw2 | #Already prevented by constraint
| 4 | '' | pw2 | #Currently allowed, but needs to be prevented

Answer 1

Use a check constraint:
CREATE TABLE foobar(
x TEXT NOT NULL UNIQUE,
CHECK (x <> '')
);
INSERT INTO foobar(x) VALUES('');

Answer 2

You can use the standard SQL 'CONSTRAINT...CHECK' clause when defining table fields:
CREATE TABLE test
(
nonempty VARCHAR NOT NULL UNIQUE CONSTRAINT non_empty CHECK(length(nonempty)>0)
)

Answer 3

As a special kind of constraint, you can put the datatype+constraint into a DOMAIN:
-- set search_path='tmp';
DROP DOMAIN birthdate CASCADE;
CREATE DOMAIN birthdate AS date DEFAULT NULL
CHECK (value >= '1900-01-01' AND value <= now())
;
DROP DOMAIN username CASCADE;
CREATE DOMAIN username AS VARCHAR NOT NULL
CHECK (length(value) > 0)
;
DROP TABLE employee CASCADE;
CREATE TABLE employee
( empno INTEGER NOT NULL PRIMARY KEY
, dob birthdate
, zname username
, UNIQUE (zname)
);
INSERT INTO employee(empno,dob,zname)
VALUES (1,'1980-02-02', 'John Doe' ), (2,'1980-02-02', 'Jon Doeh' );
INSERT INTO employee(empno,dob,zname)
VALUES (3,'1980-02-02', '' ), (4,'1980-01-01', 'Joan Doh' );
This will allow you to reuse the domain again and again, without having to copy the constraint every time.
-- UPDATE 2021-03-25 (Thanks to #AlexanderPavlov)
There appears to be a serious flaw in Postgres's implementation: it is possible to insert NULLs from the results of an empty scalar subquery.
The (nonsensical) COALESCE() below "fixes" this behaviour.
This allows us to put the database into a forbidden state.
\echo literal NULL
INSERT INTO employee(empno,dob,zname) VALUES (5,'2021-02-02', NULL );
\echo empty (scalar) set
INSERT INTO employee(empno,dob,zname) VALUES (6,'2021-02-02', (select zname from employee where 1=0) );
\echo empty COALESCE((scalar, NULL) ) set
INSERT INTO employee(empno,dob,zname) VALUES (7,'2021-02-02', (select COALESCE(zname,NULL) from employee where 1=0) );
\echo empty set#2
INSERT INTO employee(empno,dob,zname) (select 8,'2021-03-03', zname from employee where 1=0 );
\echo duplicate the complete table
INSERT INTO employee(empno,dob,zname) (select 100+empno,dob+'1mon':: interval, upper(zname) from employee );
select * from employee;
Extra Results:
literal NULL
ERROR: domain username does not allow null values
empty (scalar) set
INSERT 0 1
empty COALESCE((scalar, NULL) ) set
ERROR: domain username does not allow null values
empty set#2
INSERT 0 0
duplicate the complete table
ERROR: domain username does not allow null values
empno | dob | zname
-------+------------+----------
1 | 1980-02-02 | John Doe
2 | 1980-02-02 | Jon Doeh
6 | 2021-02-02 |
(3 rows)