How do you overcome the endless maze of code in a query?

I'm rather new at working with SQL, and went from one Fortune 500 to another. Previously I worked in Access, I know, horrible right? Now I work in Oracle and SQL server. I often see these nightmare SQL queries, 1200 lines of code are more, truly horrific size and filled with tons of crazy things that are not commented, have no explanation as to why they are done, and I'm supposed to just jump in and explain why they are …

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