SQL Tips (4 to 6)

SQL tips 4:
Avoid INDEX, unless you need to retrieve information quickly. Index will slower insert and update data query.

The another way is using sub query

Select MAX(salary)
FROM employee
WHERE salary IN
(Select top 2 salary from employee where salary is not null order by salary desc)

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SQL tips 5:
NULL and ZLS (zero length string) both are different.

NULL means data is unknown or missing.

NULL should be checked with IS NULL or IS NOT NULL only. Comparison operator equal to (=) or not equal to (<>) should not be used.

ZLS should be checked with two single quotes  ('') together.                        

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SQL tips 6:
Always use primary key with integer field and unique key with string or text field.

This is not rule, it is best practice.