Alternatively IN with EXISTS, NOT EXISTS with alternative NOT IN
Avoid the use of columns in the index calculation, because this will cause the index fail
Avoid the use of IS NULL and IS NOT NULL index columns
Query optimization, should try to avoid full table scan, first consider the establishment by the column involved in where and order index
Try to avoid fields where the null value judgment clause, or your engine will give up using the index, so a full table scan
Try to avoid the operation of the fields in the where clause expression, or your engine will give up using the index, so a full table scan
Try to avoid using the where clause! = Or <> operator, or they will give up the engine using the index, thereby performing a full table scan
Try to avoid the use or to connect to the conditions in the where clause, or your engine will give up using the index, so a full table scan
If the value is continuous, you can not use in between
Do not use anywhere select * from table, use the specific field instead of "*", remove only the required fields
Fuzzy searching try to avoid using pre-percent sign, or your engine will give up using the index, so a full table scan
An index number table is best not more than 6, because too much will affect the index update speedometer
If the field contains only numerical information, try not designed for the type of character, otherwise it will reduce the performance of queries and connections, while also increasing the storage overhead
Avoid frequent create and delete temporary tables, system tables to reduce the consumption of resources
Try to use varchar / nvarchar instead of char / nchar, because of the small field of variable-length memory space, it saves storage space, while for the query, the search within a relatively small field, more efficient naturally
Try to use table variables instead of temporary tables
