Can the NOT operator work on multiple conditions present in parenthesis?

Question

I run following query in MySQL :

SELECT * FROM Customers
WHERE NOT Country='Germany' AND NOT Country='USA';

It returned me exactly the correct expected result.

After that I run following query :

SELECT * FROM Customers
WHERE NOT (Country='Germany' AND Country='USA');

This time it returned me all the records present in the table(result-set of the query SELECT * FROM Customers;)

I'm not understanding why the NOT operator is not working in second query with parenthesis?

You can go to this link and check :

https://www.w3schools.com/sql/trysql.asp?filename=trysql_select_where_not_and

Answer 1

You got bitten by operator priority. Parentheses are your friend, you see.

WHERE NOT Country='Germany' AND NOT Country='USA';

actually means, since NOT has precedence over AND:

WHERE (NOT Country='Germany') AND (NOT Country='USA');

Since "NOT a=b" is "a!=b", this means:

WHERE Country!='Germany' AND Country!='USA';

which, as the others pointed out, looks muuuuch better like this:

WHERE Country NOT IN ('Germany', 'USA');

Now, next topic. This one:

WHERE NOT (Country='Germany' AND Country='USA');

Country can not be Germany and USA at the same time, so the stuff inside the () is always FALSE, so this equates to:

WHERE NOT (FALSE);

i.e.

WHERE TRUE;

ie, you get your whole table.