How to populate a table’s foreign keys from other tables

I’ve got the following tables, of which translation is empty and I’m trying to fill:

translation {
    id
    translated
    language_id
    template_id
}

language {
    id
    langname
    langcode
}

template {
    id
    tplname
    source
    domain
    total
}

The source data to fill translation is a temporary table that I’ve populated from an external CSV file:

tmp_table {
    id
    translated
    langname
    tplname
    source
    domain
}

What I’d like to do is to fill translation with the values from tmp_table. The translated field can be copied directly, but I’m not quite sure how to fetch the right language_id (tmp_table.langname could be used to determine language.id) and template_id (tmp_table.tplname, tmp_table.source, tmp_table.domain together can be used to determine template.id).

It might be a trivial question, but I’m quite new to SQL and not sure what the best query should be to populate the translation table. Any ideas?

Answer

This can be simplified to:

INSERT INTO translation (id, translated, language_id, template_id)
SELECT tmp.id, tmp.translated, l.id, t.id
FROM   tmp_table tmp
JOIN   language l USING (langname)
JOIN   template t USING (tplname, source, domain)
ORDER  BY tmp.id;

I added an ORDER BY clause that you don’t strictly need, but certain queries may profit if you insert your data clustered like that (or some other way).

If you want to avoid losing rows where you can’t find a matching row in language or template, make it LEFT JOIN instead of JOIN for both tables (provided that language_id and template_id can be NULL.

In addition to what I already listed under your previous question: If the INSERT is huge and constitutes a large proportion of the target table, it is probably faster to drop all indexes on the target table and recreate them afterwards. Creating indexes from scratch is a lot faster then updating them incrementally for every row.

Unique indexes also serve as constraints, so you’ll have to consider whether to enforce the rules later or leave them in place.