1. The date attribute column does not slow down the query because of the difference in minutes and seconds
2. when you use LIKE for query, if the mode starts with a specific string such as "abc %", using an index will improve efficiency. If the mode starts with a wildcard such as "% xyz, the index does not work.
3. OR may cause a full table scan, which is equivalent to IN.
4. Use NOT as few as possible
5. The execution efficiency of EXISTS and IN is the same.
6. The usage efficiency of the charindex () function is the same as that of the LIKE function with the wildcard %.
7. UNION is not necessarily more efficient than OR.
8. Field extraction should follow the principle of "How much is required and how much is requested" to avoid "SELECT *"
9. COUNT (*) is no slower than COUNT (field)
10. order by sorting BY clustered index columns with the highest efficiency
11. Use "TOP" for data extraction to improve efficiency