Join Types

A lot of real-world reporting errors — missing rows, doubled counts, totals that don't add up — trace back to merging two tables with the wrong join type. This chart makes that concrete: two small tea-club tables — members and their orders — get joined together. Pick a join type below and watch which rows survive, and where NULLs appear. A few other named join types exist (semi-joins, anti-joins, and the like) — they behave like the ones here with an extra filter layered on top, so they're left out here to keep things focused.

Member ID is the join key, highlighted in every table. Rows that don't make it into the output dim out below.

What this teaches: Pick an inner, left, right or full join on two small tables and see which rows survive and where NULL values appear. It teaches how SQL join types work and how the wrong join produces missing rows, doubled counts and totals that don't add up.