Picking The Wrong SQL Join Key Cost Me Weeks Of Accurate Data. Don’t Let It Happen To You.

Even if you’re choosing the correct SQL JOIN, you could still make a tiny mistake that could cost you — or your org — big time.

Zach Quinn

— 6 min read
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Photo by Elisa Ventur on Unsplash

Debugging A SQL JOIN Key Mistake

One of my most frustrating SQL debugging sessions at work didn’t involve overhauling a CTE or refactoring a user defined function; instead, it all came down to one function: LOWER().

In this instance even though I chose the correct JOIN to merge the necessary tables, I overlooked a critical and undervalued component of the JOIN: The join key, or column, I had chosen to match the rows in each table.

Because I was working with STRING types, there was the possibility that the data would include STRING representations in sentence case, all lower and capital forms.

By not applying a function that would make the values uniform, I was omitting data that couldn’t be matched and, by extension, producing an output that suggested our latest change wasn’t working.

After myself, fellow engineers and even our SQL-inclined management took a look, someone finally suggested LOWER() as the welcomed but also frustratingly simple fix.

A lot of SQL education focuses heavily on JOIN relationships: INNER, OUTER, LEFT, RIGHT, etc. But in doing so, misses an opportunity to teach what I believe to be an equally important focus: Understanding your JOIN keys and applying the right choice.

Because if you don’t begin with an understanding of the relationships between your table’s data, no JOIN combination can out-engineer a shaky and illogical foundation.

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