(Oracle DB) - 6.1. UNIQUE

/* UNIQUE */
	-- UNIQUE is an object of a database
	-- A value of a data in a column MUST BE only unique value in the column

CREATE TABLE ex2_7(
	col_unique_null VARCHAR2(10) UNIQUE,
	col_unique_not_null VARCHAR2(10) UNIQUE NOT NULL,
	col_unique VARCHAR2(10),
	CONSTRAINT unique_nm1 UNIQUE (col_unique)	-- Specify 'col_unique' column to be UNIQUE
);

SELECT
	constraint_name,		-- unique_nm1 is a name
	constraint_type,
	table_name,
	search_condition
	FROM user_constraints
WHERE table_name = 'EX2_7'
;

INSERT INTO ex2_7
	VALUES
	('AA', 'AA', 'AA')
;

INSERT INTO ex2_7
	VALUES
	('AA', 'AA', 'AA')	-- ERROR : unique violated! To all the columns
;

INSERT INTO ex2_7
	VALUES
	('', 'BB', 'BB')	-- NULL can be inserted into the unique column
;

INSERT INTO ex2_7
	VALUES
	('', 'CC', 'CC')	-- NULL can be inserted again into the unique column
;