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This article mainly explains "what problems will be caused by too long SQL sentences in Mysql". Interested friends may wish to have a look. The method introduced in this paper is simple, fast and practical. Next, let the editor take you to learn "what problems will be caused by too long SQL sentences in Mysql"?

Problems caused by too long Mysql SQL statements

I am now doing data statistics, the background database is using MYSQL. There is a common requirement to query the access PV of unordered users (the unique identity of the user is the user's mobile phone number)

The first way to write:

SELECT COUNT (1) FROM day_resinlog_2012_06_12 WHERE mobile_number NOT IN

(SELECT DISTINCT mobile_number FROM tbl_cartoon_order)

The second way to write: www.2cto.com

A List combination is obtained through SELECT DISTINCT mobile_number FROM tbl_cartoon_order, which splices the collection of mobile phone numbers into a string when the xxxxx in SELECT COUNT (1) FROM day_resinlog_2012_06_12 WHERE mobile_number NOT IN (xxxxx)

Comment: the first method of writing uses a subquery, which is relatively slow, which is generally the reason why we do not use it, and why we choose the second way of writing.

On the surface, there is no problem with the second way of writing, but in fact, there is a potential risk that if we order millions of mobile phone numbers, it may cause the server to report an error. Because if a SQL is too long to exceed the amount of max_allowed_packet in the MYSQL server configuration file (my.ini), it will report an error. The solution is to change the amount of max_allowed_packet.

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