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From:Ed Carp Date:October 18 2002 6:42am
Subject:RE: about use Dev-cpp and C++
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> what's nested SQL?
> can you tell me more about it?

Nested SQL allows you to do all kinds of interesting things in MySQL that
you can't do now - sub-selects, for example; or if you're a lazy programmer
(like me!) you want to do grouping and sorting in ways that would be
difficult (and complicated) with an ordinary SQL query.

For example, suppose you had two tables - a users table and a messages
table.  You suspect that you've got orphaned records in the messages table,
but you're not sure.  You also want to do other stuff to each message as
you go through it.  So, you decide that you want to iterate through each
record in the users table, setting a flag in the messages table for each
message that isn't orphaned:

char cmd[256];

/* you could also do an ExecuteSQL() here, too */
OpenRecordset("update messages set OK='N'");
CloseRecordset();
/* or ExecuteSQL ("update messages set OK='N'"); */
OpenRecordset("select * from users");
while(RecordsetEOF() != EOF)
{
	/* ID in the users table has a one-to-many relationship to usersID in the
messages table */
	sprintf(cmd, "update messages set OK='Y' where usersID=%s",
GetField("ID"));
	OpenRecordset(cmd);
	CloseRecordset();
	/* could also do a ExecuteSQL(cmd); */
	MoveNext();
}
CloseRecordset();
/* now, we want to see which records are orphaned */
OpenRecordset("select * from messages where OK='Y'");
while(RecordsetEOF() != EOF)
{
	/* do whatever here */
	MoveNext();
}
CloseRecordset();

Of course, this is a simple example.  PLEASE DON'T POST QUERIES SHOWING HOW
THIS COULD BE DONE EASIER OR FASTER OR WHATEVER IN SQL!  I'm sure you can
think of a lot of other examples where doing a sub-select would be very
cool, but MySQL doesn't support it.

sql, query

Thread
about use Dev-cpp and C++wang tianyi14 Oct
  • Re: about use Dev-cpp and C++Insanely Great14 Oct
  • Re: about use Dev-cpp and C++Michael T. Babcock16 Oct
    • RE: about use Dev-cpp and C++Ed Carp16 Oct
      • RE: about use Dev-cpp and C++wang tianyi18 Oct
        • RE: about use Dev-cpp and C++Ed Carp18 Oct
          • RE: about use Dev-cpp and C++wang tianyi18 Oct
    • Re: about use Dev-cpp and C++wang tianyi18 Oct