SQL Tutorial: GROUP BY, JOINs, Window Functions & Indexes

WHERE drops rows before grouping. HAVING drops groups after. One chapter on the clause order, the aggregates WHERE cannot see, and the query that should have filtered earlier.

What you will learn

  • WHERE filters rows; HAVING filters groups
  • Rows, pages, and what an index actually stores
  • SELECT plans: seq scan vs index vs covering
  • Transactions, isolation, and lost updates
  • Normalization vs the query you have to serve
  • Migrations without locking the product
  • When Postgres is enough and when it is not

How it works

Broken queries and lost updates first. Then the schema, index, and isolation level that make the same write safe.

Chapters

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