Interview Q&A / SQL & DBMS

1. What is the difference between a primary key, a unique key and a foreign key?

   A primary key uniquely identifies each row and cannot be NULL; a table has only one. A unique key also prevents duplicates, but a table can have many and they usually allow NULL. A foreign key is a column that must match a primary or unique key in another table, which keeps related data consistent.

   BeginnerIn-depth3 min read
2. What is a composite key and when do you need one?

   A composite key is a key made of two or more columns that together identify a row. It is needed when no single column is unique, such as a student and subject pair in a marks table.

   BeginnerQuick answer1 min read
3. What is the difference between a super key, a candidate key and a primary key?

   A super key is any set of columns that uniquely identifies a row. A candidate key is a minimal super key with no removable column. The primary key is the one candidate key you choose to identify rows.

   BeginnerQuick answer1 min read