Adds MCQs.

database/mcqs/02-integrity-er-diagram.md

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+# Questions
+1. Joe Bloggs is helping us build Scaler. He is trying to delete some batches but the database throws an error that the batch is used by a tuple in another table. Which type of integrity constraint is being enforced here?
+    >A. User-defined integrity \
+    B. Domain integrity \
+    C. Entity integrity \
+    D. Referential integrity
+---
+2. Which of the following is not an example of domain integrity constraints?
+
+    >A. Validating that input is correct data type \
+    B. Ensuring value is not null if a non-nullable column \
+    C. Making sure each row is unique \
+    D. Checking if the value exceeds maximum length
+---
+3. Joe Bloggs is taking some time off to build his house. A house can have many rooms. What is the cardinality of house to room?
+    >A. One to Many (1:m) \
+    B. One to One (1:1) \
+    C. Many to Many (m:n)
+---
+4. Let us say a house has a house_id and a room has room_id.
+Given the cardinality of house to room, which is the ideal place to store their relationship i.e. reference?
+    >A. room_id in house \
+    B. Separate table for room_id and house_id \
+    C. Array of room_ids in house \
+    D. house_id in room
+
+---
+# Answers
+1. D. `Referential integrity`
+2. C `Making sure each row is unique`
+3. A `One to Many (1:m)`
+4. D `house_id in room`