Question

Suppose I have a student table contains id, class, school_id having 1000 records.

There are 3 schools and 12 classes.

Which of these 2 queries would be faster(if there is a difference)

Query 1:

SELECT * FROM student WHERE school = 2 and class = 5; 

Query 2:

SELECT * FROM student WHERE class = 5 and school = 2;

Note: I just changed the places of the 2 conditions in WHERE.

Then which will be faster and Is the following true?

->probable number of records in query1 is 333

->probable number of records in query2 is 80.

Answer 1

hypothetical; to teach a DB concept

"How your DB uses cardinality to optiize your queries"

So, it’s basically true that they are identical, but I will mention one thought hinting at the "why" which will actually introduce a good RDBMS concept.

Let’s just say hypothetically that your RDBMS used the WHERE clauses strictly in the order you specified them.

In that case, the optimal query would be the one in which the column with maximum cardinality was specified first. What that means is that specifying class=5 first would be faster, as it more quickly eliminates rows from consideration, meaning if the row’s "class" column does not contain 5 (which is statistically more likely than it’s "school" column not containing 2), then it doesn’t even need to evaluate the "school" column.

Coming back to reality, however, you should know that almost all modern relational database management systems do what is called "building a query plan" and "compiling the query". This involves, among other things, evaluating the cardinality of columns specified in the WHERE clause (and what indexes are available, etc). So essentially, it is probably true to say they are identical, and the number of results will be, too.