MySQL IF語句？

Question

我有一個很大的大型sql查詢，在其中選擇並格式化一堆行以導出到Excel。 我想再添加一個角色，如果已刪除（角色ID）EXISTS和IS 3，則將其設置為Role 1，否則將其設置為Role 2。

當前的mysql查詢

SELECT uc_orders.billing_first_name AS 'First Name', uc_orders.billing_last_name AS 'Last 
Name', users.name AS 'Username', uc_orders.billing_street1 AS 'Address', 
uc_orders.billing_city AS 'City', uc_zones.zone_name AS 'State',uc_orders.billing_postal_code
 AS 'Postal Code', ROUND(uc_orders.order_total,2) AS 'Order Total'  FROM uc_orders, users,
 uc_zones WHERE uc_orders.uid!='0' AND uc_orders.uid = users.uid AND uc_orders.billing_zone =
 uc_zones.zone_id AND uc_orders.modified BETWEEN '1271124575' AND '1274978899'

但是用戶將永遠不會存在於users_role數據庫中（僅當它為3或其他一些數字時才存在，通常不會存在）。 因此，如果它不存在，它還將返回“角色2”；

所以

  IF users.uid EXISTS IN users_role.uid THEN
    IF users_role.uid EQUALS users.uid AND users_role.rid = 3
      THEN
        return 'Role 1'
      ELSE
        return 'Role 2';
  ELSE return 'Role 2';

如何在MySQL中做到這一點？ 還是PHP是唯一的方法？

Answer 1

您應該對多表查詢使用JOIN 。 另外，請考慮設置查詢格式和使用別名，以便於閱讀。

SELECT
    `a`.`billing_first_name` as `First Name`,
    `a`.`billing_last_name` as `Last Name`,
    `b`.`name` as `Username`,
    `a`.`billing_street1` as `Address`,
    `a`.`billing_city` as `City`,
    `c`.`zone_name` as `State`,
    `a`.`billing_postal_code` as `Postal Code`,
    ROUND(`a`.`order_total`,2) as `Order Total`,
    IF(`d`.`rid` IS NOT NULL AND `d`.`rid`=3,'Role 1','Role 2') as `Role`
FROM `us_orders` as `a`
JOIN `users` as `b` ON `a`.`uid`=`b`.`uid`
JOIN `uc_zones` as `c` ON `a`.`billing_zone`=`c`.`zone_id`
LEFT JOIN `users_role` as `d` on `a`.`uid`=`d`.`uid`
WHERE `a`.`modified` BETWEEN '1271124575' AND '1274978899'

回答您實際問題的部分是帶有IF的行和帶有LEFT JOIN 。

Answer 2

使用CASE語句。 CASE(rid) WHEN 3 THEN 'Role 1' ELSE 'Role 2' END;

有關CASE語句的信息可以在這里找到

    -- Kolinks SQL using a CASE statement
    SELECT
        `a`.`billing_first_name` as `First Name`,
        `a`.`billing_last_name` as `Last Name`,
        `b`.`name` as `Username`,
        `a`.`billing_street1` as `Address`,
        `a`.`billing_city` as `City`,
        `c`.`zone_name` as `State`,
        `a`.`billing_postal_code` as `Postal Code`,
        ROUND(`a`.`order_total`,2) as `Order Total`,
        CASE (`d`.`rid`) WHEN 3 THEN 'Role 1' ELSE 'Role 2' END; as `Role`
    FROM `us_orders` as `a`
    JOIN `users` as `b` ON `a`.`uid`=`b`.`uid`
    JOIN `uc_zones` as `c` ON `a`.`billing_zone`=`c`.`zone_id`
    LEFT JOIN `users_role` as `d` on `a`.`uid`=`d`.`uid`
    WHERE `a`.`modified` BETWEEN '1271124575' AND '1274978899'

Answer 3

是的，有一個IF語句。 還有一個IF（）函數。 這是IF語句的MySQL參考頁：

http://dev.mysql.com/doc/refman/5.0/zh-CN/if-statement.html