Mysql SELECT，查找行数，然后在单个查询语句中INSERT

Question

Let say we have inbox table and only maximum 50 messages can be recieved at a time, I use the following code for this purpose:

$maximum_messages_in_inbox = 50;

$query1 = mysql_query("SELECT * from inbox");
if(mysql_num_rows($query1) > $maximum_messages_in_inbox)
{
   echo "Message memmory if full please try again later";
}
else
{
   mysql_query("insert into inbox set text = '$text' , sender = '$sender' ");
}

This code work very well, but each time two queries will be executed.
My question is can we adjust above purpose in a single query like:

mysql_query("select * from inbox if num > 50 insert else error");
if(error)
{
   echo "Message memmory if full please try again later";
}

Looking forward to your co-operation. Thanks

Answer 1

You can do conditional INSERT in SQL with this trick :

INSERT INTO inbox(text, sender)
SELECT 'text value', 'sender value'
FROM dual
WHERE (SELECT COUNT(*) FROM inbox) < 50;

Thay way the INSERT will be processed only if the WHERE clause is satisfied.