Dear all,
A table "book" looks like this:
"page" "content" -------------- 1 abc 1 de 2 fgh 3 ijk 3 lmn 3 opq ... 210 z
I need a query to randomly select 90% of all pages and their corresponding contents.
I have written the following querys. But I have problems to combine them into a single one. (I use mysql.)
// Randomly select 40 pages of the book. Query1: SELECT DISTINCT page FROM tbl ORDER BY RAND() LIMIT 40;
// Count the total pages of the book. But how to use this value in the LIMIT clause in the above query to indicate 90% of all pages? Query2: SELECT COUNT(DISTINCT page) FROM book;
Using nothing but SQL, you cannot. You could select the value of COUNT() into a SQL variable, like this:
SELECT @c := COUNT(DISTINCT page) FROM book;
But you cannot use @c in a LIMIT clause, because the argument to LIMIT must be an integer constant.
If you issue your queries from within a script, you can use the programming language to get the COUNT() value, and then insert that value into the next query so that it appears as an integer constant to MySQL.
// How to embed/alias Query1 in Query3? Query3: SELECT * FROM book WHERE page IN Q1;
Thanks a lot.
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