Question

Is it possible to insert a new row if a condition is meet?

For example, i have this table with no primary key nor uniqueness

    +----------+--------+
    | image_id | tag_id |
    +----------+--------+
    |    39    |    8   |
    |    8     |    39  |
    |    5     |    11  |
    +----------+--------+

I would like to insert a row if a combination of image_id and tag_id doesn’t exists
for example;

INSERT ..... WHERE image_id!=39 AND tag_id!=8

Answer 1

I think you’re saying: you need to avoid duplicate rows in this table.

There are many ways of handling this. One of the simplest:

INSERT INTO theTable (image_id, tag_id) VALUES (39, 8)
WHERE NOT EXISTS 
    (SELECT * FROM theTable 
    WHERE image_id = 39 AND tag_id = 8)

As @Henrik Opel pointed out, you can use a check constraint on the combined columns, but then you have to have a try/catch block somewhere else, which adds irrelevant complexity.

Edit to explain that comment…

I’m assuming this is a table mapping a many-to-many relationship between Movies and Tags. I realize you’re probably using php, but I hope the C# pseudocode below is clear enough anyway.

If I have a Movie class, the most natural way to add a tag is an AddTag() method:

class Movie 
{
    public void AddTag(string tagname)
    {
        Tag mytag = new Tag(tagname); // creates new tag if needed

        JoinMovieToTag(this.id, mytag.id);
    }

    private void JoinMovieToTag(movie_id, tag_id)
    {
        /* database code to insert record into MovieTags goes here */

        /* db connection happens here */
        SqlCommand sqlCmd = new SqlCommand("INSERT INTO theTable... /* etc */");

        /* if you have a check constraint on Movie/Tag, this will 
           throw an exception if the row already exists */
        cmd.ExecuteNonQuery(); 
    }
}

There’s no practical way to check for duplicates earlier in the process, because another user might Tag the Movie at any moment, so there’s no way around this.

Note: If trying to insert a dupe record means there’s a bug, then throwing an error is appropriate, but if not, you don’t want extra complexity in your error handler.