The plain-English version
The relationship between the number of matching records on each side of a join.
Why it matters
It determines whether combining tables preserves, removes, or multiplies rows.
Let’s make it click
Joining orders to customers should usually match many orders to one customer record.
Vibe Bro’s heads-up
Duplicate customer keys can multiply orders and inflate revenue; check key uniqueness and row counts before and after joining.
The docs behind it
Documentation links checked .
Editorial review · October 28, 2026
Supported: Checked pandas' many-to-many Cartesian-product behavior; duplicated join keys can multiply source rows.