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> SQL expresses relational algebra Actually, no. Relational algebra is based on sets. SQL is not. Perhaps you are confusing SQL with QUEL, which was used by Po

• matwood 4y ago
  How is SQL not based on sets? One of the big issues I see when people first come to SQL is not thinking in sets. Instead they think in individual records and loops, which is the exact opposite on how to think about SQL.
  • randomdata 4y ago
    Sets are collections of unique elements, and indeed relations are sets of tuples. But SQL only sticks to the tuples part, foregoing sets. It's quite possible, and quite likely if you're not careful, to end up with duplicate entires in your results to an unwanted effect. SQL is only relational-like. With experience and care you can craft your queries in such a way that you do get true sets (e.g. using DISTINCT), but I see beginners – and sometimes even experts in a rush – overlook this quirk in a complex query resulting in what might seem okay in limited testing, but blows up when there is more data than the bare minimum needed to test. A language that truly follows Codd's relational model would produce sets always. I expect SQL is considered difficult to learn exactly because of this deviation. It's quite unintuitive, especially if you go in thinking that it is actually relational. Nothing you can't work around with sufficient knowledge and experience, but that added knowledge and experience required is where the difficulty no doubt stems from and undoubtedly scares many away out of frustration when it doesn't work like you think it should in the interim.