チェック制約で文字列長を制限する

check_constraint_sample.sql

CREATE TABLE check_constraint_sample (col_not_null VARCHAR(255) NOT NULL, col_nullable VARCHAR(255));

ALTER TABLE check_constraint_sample ADD CONSTRAINT constraint1 CHECK (LENGTH(col_not_null) > 0);
ALTER TABLE check_constraint_sample ADD CONSTRAINT constraint2 CHECK (LENGTH(col_nullable) > 0);

-- 正常終了する
INSERT INTO check_constraint_sample (col_not_null, col_nullable) VALUES ('not null', 'not null');
INSERT INTO check_constraint_sample (col_not_null, col_nullable) VALUES ('not null', null);

-- 以下、すべてエラー
INSERT INTO check_constraint_sample (col_not_null, col_nullable) VALUES ('not null', '');
INSERT INTO check_constraint_sample (col_not_null, col_nullable) VALUES (null, 'not null');
INSERT INTO check_constraint_sample (col_not_null, col_nullable) VALUES ('', 'not null');

PostgreSQL 9.6 系でとりあえず動いた（手元に 9 系しか入ってなかった…）