How can you compare if two databases are equal using SQL?

• A. Select * from tableA intersect select * from tableB
• B. select * from tableA plus select * from tableB
• C. Select * from tableA minus select * from tableB
• D. select * from tableA join tableB

Answer: (C)

To compare if two databases (or tables within them) are equal using SQL, the correct approach involves ensuring that all records present in one table are also found in the other, without any discrepancies. Selecting from tableA and subtracting from tableB (using the "minus" operation) effectively identifies records that are in tableA but not in tableB.

When you use the "minus" function, you are filtering out all rows in tableA that exist in tableB. If the result returns any rows, it indicates that tableA has additional records not present in tableB, suggesting that the two tables are not equal. This can be further cross-verified by performing the operation in the opposite direction - subtracting tableB from tableA. If both operations yield no results, it confirms that the two tables are indeed the same.

This approach highlights the need for a comprehensive check by not only confirming that elements of tableA are in tableB but also vice versa, ensuring equality rather than just a one-way check. This contrasts with options that either focus solely on intersection or inappropriate combinations like "plus" or "join," which do not directly address the equality comparison between two datasets.