Practical SQL — From Queries to Performance

About this course

Most SQL courses stop at SELECT * FROM users WHERE id = 5. We start there and don't stop until you can write a query that pulls a 30-day rolling cohort retention curve from a 50M-row events table — and reads its EXPLAIN plan to know it'll run in under a second. Postgres throughout (the dominant database in MENA tech).

What you'll cover

• 1

  Postgres setup and the sample dataset

  Install. Load a real schema (users, events, orders). Your first SELECTs.

• 2

  JOINs — INNER, LEFT, and the ones you'll forget

  When each is right. Avoiding cartesian explosions. Multi-table joins.

• 3

  Aggregates, GROUP BY, and HAVING

  COUNT, SUM, AVG. The GROUP BY mental model. WHERE vs HAVING.

• 4

  Window functions — the analyst's superpower

  ROW_NUMBER, RANK, LAG, running totals. The queries that make you look senior.

• 5

  CTEs vs subqueries

  Readable layered queries with WITH. When CTEs help and when they hurt.

• 6

  Indexes and EXPLAIN

  B-tree, GIN, partial indexes. Reading a query plan. Why your query is slow.

• 7

  Real business queries — retention, funnel, cohort

  30-day retention. Conversion funnel. Cohort analysis. The queries every PM asks for.

Skills you'll build

• SQL
• PostgreSQL
• JOINs
• window functions
• EXPLAIN
• indexes
• CTEs
• data analysis