如何创建即使列为NULL仍然有效的UNIQUE约束

Question

I have a table which I want to prevent duplicate rows from being inserted on.

CREATE TABLE blah(id INTEGER PRIMARY KEY ASC, a INTEGER, b INTEGER, c, d, e, f,
    UNIQUE (a,b,c,d,e,f) ON CONFLICT IGNORE);

Now I insert a couple rows:

INSERT INTO event2 (a,b,c,d) 
    VALUES (1,1405451500,"Y",100), 
           (1,1405451555,"Z",115);

Now if I re-run the above command, I would expect it to not insert anything, because the new rows exactly match what is already there. Instead, I see two new rows inserted.

Reading the SQLite documentation , it appears that SQLite does not enforce UNIQUE constraints if one or more of the columns are NULL. If there some way to achieve this goal using some other mechanism? In the above case, the c , d , e , and f columns could be null. Alternatively I could use empty strings instead of NULL but reading some other answers seemed to suggest that null shouldn't be confused with the empty string as they denote separate things.

Answer 1

You can use BEFORE INSERT trigger to check data correctness.

Be aware, that this does full table scan before every insert, so for bigger tables it will cause inserts to be slow!

To speed it up (and avoid full table scans) you should CREATE INDEX , which will include all columns checked in your trigger.

Here's how you do it with your example:

CREATE TRIGGER event2uniqTrigger
       BEFORE INSERT ON event2
       WHEN ( 
    SELECT count( * )
      FROM event2 t
     WHERE ( CASE
                    WHEN new.a IS NULL THEN t.a IS NULL 
                    ELSE t.a = new.a 
           END ) 
           AND
           ( CASE
                    WHEN new.b IS NULL THEN t.b IS NULL 
                    ELSE t.b = new.b 
           END ) 
           AND
           ( CASE
                    WHEN new.c IS NULL THEN t.c IS NULL 
                    ELSE t.c = new.c 
           END )  
           AND
           ( CASE
                    WHEN new.d IS NULL THEN t.d IS NULL 
                    ELSE t.d = new.d 
           END )  
           AND
           ( CASE
                    WHEN new.e IS NULL THEN t.e IS NULL 
                    ELSE t.e = new.e 
           END )  
           AND
           ( CASE
                    WHEN new.f IS NULL THEN t.f IS NULL 
                    ELSE t.f = new.f 
           END )  
    ) > 0
BEGIN
    SELECT RAISE ( FAIL, 'inserted value not unique' );
END;

and the index:

CREATE INDEX event2idx ON event2 (a, b, c, d, e, f);

You may also want to create another trigger for BEFORE UPDATE . It would look just like the one above, except the BEFORE UPDATE part.