Provide syntactic sugar for upserting a record

[isuru89]

Currently if a user wants to insert if not exist or update (upsert) a particular record to a table, he/she must write a script to do it which having at least three queries; one for check the existence, other for inserting, and another for updating.

It is nice to provide an upsert syntax to do this is one single expression (or query). AFAIK there are no rdbms database having a single syntax for upserting, but it is believed NyQL can do this by simulating these three steps automatically.

Need to think about how the syntax would like be.

[isuru89]

Basically syntax for an upsert query would be as same as an UPDATE query.
i.e. it has;

• TARGET table
• Optional JOINING tables
• SET clause
• WHERE clause

In addition to that, user can specify what to return after the execution of an upsert query. Such as, the record before update/insert happens, record after update/insert, custom fetched columns after update/insert, or nothing. These values may be defined using clauses RETURN_BEFORE, RETURN_AFTER, RETURN_COLUMNS, RETURN_NONE respectively.

Eg: Upsert query would look like

$DSL.upsert {
    TARGET (...)
    JOIN { ... }     // optional
    SET { ... }
    WHERE { ... }

    RETURN_AFTER()   // new clause user has to specify what to return
}