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The SESSION has already started $ username = $ _ SESSION ['username']; mysql_query (& quot; updateuserman. userssetmeinv = (selectxueyuanfromtest. meinvwherei: What are the errors in this statement?
The SESSION has been started.

$ Username = $ _ SESSION ['username'];
Mysql_query ("update userman. users set meinv = (select xueyuan from test. meinv where id = $ id) where username = $ username ;");

No error reported, but the database is not updated

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Mysql_query ("update userman. users set meinv = (select xueyuan from test. meinv where id = $ id) where username = '$ username'") or die (mysql_error ());

Let's take a look.
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Does your session have a value?

Echo $ username; // output