“Insert if not exists” statement in SQLite

问题:

I have an SQLite database. I am trying to insert values (users_id, lessoninfo_id) in table bookmarks, only if both do not exist before in a row.

INSERT INTO bookmarks(users_id,lessoninfo_id) 
VALUES(
    (SELECT _id FROM Users WHERE User=\'\"+$(\'#user_lesson\').html()+\"\'),
        (SELECT _id FROM lessoninfo 
        WHERE Lesson=\"+lesson_no+\" AND cast(starttime AS int)=\"+Math.floor(result_set.rows.item(markerCount-1).starttime)+\") 
        WHERE NOT EXISTS (
            SELECT users_id,lessoninfo_id from bookmarks 
            WHERE users_id=(SELECT _id FROM Users 
            WHERE User=\'\"+$(\'#user_lesson\').html()+\"\') AND lessoninfo_id=(
                SELECT _id FROM lessoninfo
                WHERE Lesson=\"+lesson_no+\")))

This gives an error saying:

db error near where syntax.

回答1:

If you have a table called memos that has two columns id and text you should be able to do like this:

INSERT INTO memos(id,text) 
SELECT 5, \'text to insert\' 
WHERE NOT EXISTS(SELECT 1 FROM memos WHERE id = 5 AND text = \'text to insert\');

If a record already contains a row where text is equal to \'text to insert\' and id is equal to 5, then the insert operation will be ignored.

I don\'t know if this will work for your particular query, but perhaps it give you a hint on how to proceed.

I would advice that you instead design your table so that no duplicates are allowed as explained in @CLs answer below.

回答2:

If you never want to have duplicates, you should declare this as a table constraint:

CREATE TABLE bookmarks(
    users_id INTEGER,
    lessoninfo_id INTEGER,
    UNIQUE(users_id, lessoninfo_id)
);

(A primary key over both columns would have the same effect.)

It is then possible to tell the database that you want to silently ignore records that would violate such a constraint:

INSERT OR IGNORE INTO bookmarks(users_id, lessoninfo_id) VALUES(123, 456)

回答3:

For a unique column, use this:

INSERT OR REPLACE INTO table () values();

For more information, see: sqlite.org/lang_insert

回答4:

insert into bookmarks (users_id, lessoninfo_id)

select 1, 167
EXCEPT
select user_id, lessoninfo_id
from bookmarks
where user_id=1
and lessoninfo_id=167;

This is the fastest way.

For some other SQL engines, you can use a Dummy table containing 1 record. e.g:

select 1, 167 from ONE_RECORD_DUMMY_TABLE