🔗 SQL exists, in and join checks

Database queries rely on cross table checks in many everyday reports and features. A common need is to ask if a value from one table has at least one matching row in a related table, without returning all columns from that second table. SQL gives several tools for this kind of presence check, and three of the most often used forms are EXISTS, IN, and join based filters. Each form expresses the same idea with different syntax and different effects on readability and performance.