提升MYSQL查询效率的10个SQL语句优化技巧

// query cache does NOT work

$r = mysql_query("SELECT username FROM user WHERE signup_date >= CURDATE()"); 
// query cache works! 
$today = date("Y-m-d"); 
$r = mysql_query("SELECT username FROM user WHERE signup_date >= '$today'"); 
// query cache does NOT work 
$r = mysql_query("SELECT username FROM user WHERE signup_date >= CURDATE()"); 
// query cache works! 
$today = date("Y-m-d"); 
$r = mysql_query("SELECT username FROM user WHERE signup_date >= '$today'");

// do I have any users from Alabama? 
// what NOT to do: 
$r = mysql_query("SELECT * FROM user WHERE state = 'Alabama'"); 
if (mysql_num_rows($r) > 0) { 
 // ... 
}  
// much better: 
$r = mysql_query("SELECT 1 FROM user WHERE state = 'Alabama' LIMIT 1"); 
if (mysql_num_rows($r) > 0) { 
 // ... 
}

// looking for companies in my state 
$r = mysql_query("SELECT company_name FROM users 
 LEFT JOIN companies ON (users.state = companies.state) 
 WHERE users.id = $user_id"); 
// both state columns should be indexed 

// and they both should be the same type and character encoding 

// or MySQL might do full table scans

// what NOT to do: 
$r = mysql_query("SELECT username FROM user ORDER BY RAND() LIMIT 1"); 
// much better: 
$r = mysql_query("SELECT count(*) FROM user"); 
$d = mysql_fetch_row($r); 
$rand = mt_rand(0,$d[0] - 1); 
$r = mysql_query("SELECT username FROM user LIMIT $rand, 1");

// not preferred 
$r = mysql_query("SELECT * FROM user WHERE user_id = 1"); 
$d = mysql_fetch_assoc($r); 
echo "Welcome {$d['username']}"; 
// better: 
$r = mysql_query("SELECT username FROM user WHERE user_id = 1"); 
$d = mysql_fetch_assoc($r); 
echo "Welcome {$d['username']}"; 
// the differences are more significant with bigger result sets

// create a prepared statement 
if ($stmt = $mysqli->prepare("SELECT username FROM user WHERE state=?")) { 
 // bind parameters 
 $stmt->bind_param("s", $state); 
 // execute 
 $stmt->execute(); 
 // bind result variables 
 $stmt->bind_result($username);  
 // fetch value 
 $stmt->fetch(); 
 printf("%s is from %s\n", $username, $state);  
 $stmt->close(); 
}