Using SQL functions with Storm

08Apr10

People ask me questions about Storm several times a week, and I’m happy to help where I can, but it would be better if the information our users need was easier to find. Our documentation story is weak and, as a result, Storm is harder to discover than it ought to be. If you do run into problems or have questions, please hop into #storm on Freenode and we’ll do our best to help you. I want to improve the documentation situation, but finding the time to do so hasn’t happened yet. The next best thing is to post some helpful hints here.

First, some advice: if you don’t know what SQL query you want to run, using Storm will probably be hard. When I get asked a question about writing a query using Storm I almost always respond with, “What does the query you want to run look like?” The querying part of Storm is essentially just an expression compiler that takes a list of expressions and converts them to SQL. Once you know what end result you want, working backwards to figure out the Storm parlance is relatively straight forward.

Today I was asked about using COALESCE. The person asking the question found the storm.expr.Coalesce expression but it wasn’t obvious how to use it. He knew he wanted to run a query like:

SELECT * FROM table WHERE COALESCE(col1, col2) = 'foo'

With that knowledge, figuring out how to use Storm was quite easy:

store.find(Table, Coalesce(Table.col1, Table.col2) == "foo")

Furthermore, Coalesce is just a helper expression to aid usability and readability. If we didn’t have a built-in Coalesce expression the query could have been written using the generic storm.expr.Func expression:

store.find(Table,
           Func("COALESCE", Table.col1, Table.col2) == "foo")

If you want to use a SQL function in a Storm query, and there is no built-in expression for it, you can use Func to call it.

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