Below are some tips to remember while we are writing a select query in MySQL that can help us and reduce our query time:-
Whenever we use where in a large table we should make sure the column in where clause are index or not. Ex:- Select * from employee where user_id > 2000. user_id if indexed then will speed up the evaluation of the query atlot. Indexes are also very important during joins and foreign keys.
When you need the smaller section of content rather then fetching whole data from table, try to use limit. Rather then writing Ex:- Select * from employee. If you need just first 20 employee from lakhs then just use limit Ex:- Select * from employee LIMIT 20.
You can also optimize your query by providing the column name which you want in resultset. Rather then writing Ex:- Select * from employee. Just mention column name from which you need data if you table has lots of column and you want to have data for few of them. Ex:- Select id, name from employee.
Index column if you are using to verify for NULL in where clause. If you have some statement as SELECT * FROM tbl_name WHERE key_col IS NULL; then if key_col is indexed then query will be evaluated faster.