使用數據庫計數而不是在PHP中計算結果mysqli

Question

如何轉換爲使用數據庫計數？

function get_orders_count(){                  
    global $mysqli;                   
    $query ='                      
    SELECT                     
     a.order_id                  
    FROM                     
     assignments a                 
    WHERE                     
     client_id = '.$_SESSION['user']['ID'];           
    $result = $mysqli->query($query);                
    return $result->num_rows;                  
} 

Answer 1

使用COUNT SQL函數。

function get_orders_count(){                  
    global $mysqli;                   
    $query ='                      
    SELECT                     
     COUNT(a.order_id)                  
    FROM                     
     assignments a                 
    WHERE                     
     client_id = '.$_SESSION['user']['ID'];           
    $result = $mysqli->query($query);                
    return $result->$result->fetch_row()[0];                  
} 

如果order_id是主鍵使用

COUNT(*)