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Not that I want to teach SQL to any one, but do not forget this simple things to improve your queries:

• Usage of Limi/Top if we know how many rows should be return, can save time.
• Do not return * unless you really need all the fields from the table. Saves time and improve.
• If you need a random value use order by newid() for SqlServer and order by rand() from MySql
• Do not save binary data in the rows! (like images or docs etc…)
• If you need to join two tables don’t do this: select * from table1,table2, instead use JOIN
• Important to have indexes in the most used fields.
• The where clauses try them to be for the indexed fields.
• Usage of the same query, this ones once is done is cached.
• Procedures can save lot of time for common queries.
• Try to avoid the usage of Like ‘%something%’ instead be more specific field=’something’.

And last one:

Please, if you need to return the amount of rows from a table, use count(), and never the recordest.count method.

This last one I saw it at many places and kills applications of course!

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