P-6.2 Querying With SQL

Work through these

• Selecting columns and filtering rows

  Choosing which columns to return and which rows to keep covers a surprising share of everyday queries. Being specific about columns rather than taking all of them is a habit worth forming early.

  SQLBolt: Learn SQL with interactive exercises · Tutorial

• Sorting and limiting results

  Without an explicit ordering a database is free to return rows in any order it likes, and it will change its mind between runs. Any query whose output order matters must say so.

• Missing values are not equal to anything, including themselves

  A comparison against a missing value returns neither true nor false, so rows silently drop out of results that should contain them. There is a separate test for absence and you must use it.

• Pattern matching, ranges and set membership

  Filtering by a text pattern, a numeric range or a listing of accepted values covers most remaining conditions. Knowing which of these can use an index comes later and starts here.

• Write ten queries against a real dataset without looking anything up

  Load a dataset with a few thousand rows and answer real questions about it. Fluency here is a matter of repetition rather than understanding, and it arrives quickly.

  PostgreSQL Documentation: Tutorial · Docs