在MySQL中嵌套IF的評估

Question

我在WordPress插件中發現了這個代碼，但我無法理解它是什麼意思或如何讀取它。

有人可以幫助我瞭解這個代碼：？

IF(agr_sam_ads.ad_users = 0, TRUE, IF(agr_sam_ads.ad_users_reg = 1, IF(agr_sam_ads.x_ad_users = 1, NOT FIND_IN_SET("admin", agr_sam_ads.x_view_users), TRUE .... 

我還沒有粘貼整個查詢，因爲它很大。在我感興趣得到幫助的是，看起來像查詢的一部分：

IF(expression, value, IF(expression, IF(expression, SQL Logical Query, value ... 

我還沒有看到這樣的語法，我不知道哪裏來搜索。在MySQL文檔中，IF語句的語法如下：http://dev.mysql.com/doc/refman/5.0/en/if.html，它看起來不像我上面粘貼的那個。

只爲用戶感興趣的完整的代碼，該代碼是在這裏：

SELECT 
agr_sam_places.id, 
agr_sam_places.name, 
agr_sam_places.description, 
agr_sam_places.code_before, 
agr_sam_places.code_after, 
agr_sam_places.place_size, 
agr_sam_places.place_custom_width, 
agr_sam_places.place_custom_height, 
agr_sam_places.patch_img, 
agr_sam_places.patch_link, 
agr_sam_places.patch_code, 
agr_sam_places.patch_adserver, 
agr_sam_places.patch_dfp, 
agr_sam_places.patch_source, 
agr_sam_places.trash, 
(
    SELECT 
     COUNT(*) 
    FROM 
     agr_sam_ads 
    WHERE 
     agr_sam_ads.pid = agr_sam_places.id 
    AND 
     agr_sam_ads.trash IS FALSE 
) AS ad_count, 
(
    SELECT 
     COUNT(*) 
    FROM 
     agr_sam_ads 
    WHERE 
     agr_sam_ads.pid = agr_sam_places.id 
    AND 
     agr_sam_ads.trash IS FALSE 
    AND 
     (
      IF(agr_sam_ads.ad_users = 0, TRUE, IF(agr_sam_ads.ad_users_reg = 1, IF(agr_sam_ads.x_ad_users = 1, NOT FIND_IN_SET("admin", agr_sam_ads.x_view_users), TRUE 
     ) 
    AND 
     IF(agr_sam_ads.ad_users_adv = 1, (agr_sam_ads.adv_nick <> "admin"), TRUE), FALSE))) 
    AND 
     (
      (
       agr_sam_ads.view_type = 1 
      ) 
     OR 
      (
       agr_sam_ads.view_type = 0 
      AND 
       (
        agr_sam_ads.view_pages+0 & 256 
       ) 
      ) 
     ) 
    AND 
     (
      agr_sam_ads.ad_cats = 0 
     ) 
    AND 
     (
      agr_sam_ads.ad_authors = 0 
     ) 
    AND 
     IF(agr_sam_ads.ad_schedule, CURDATE() BETWEEN agr_sam_ads.ad_start_date AND agr_sam_ads.ad_end_date, TRUE) 
    AND 
     IF(agr_sam_ads.limit_hits, agr_sam_ads.hits_limit > agr_sam_ads.ad_hits, TRUE) 
    AND 
     IF(agr_sam_ads.limit_clicks, agr_sam_ads.clicks_limit > agr_sam_ads.ad_clicks, TRUE) 
    AND 
     (
      agr_sam_ads.ad_weight > 0 
     ) 
) AS ad_logic_count, 
(
    SELECT 
     COUNT(*) 
    FROM 
     agr_sam_ads 
    WHERE 
     agr_sam_ads.pid = agr_sam_places.id 
    AND 
     agr_sam_ads.trash IS FALSE 
    AND 
     (
      IF(agr_sam_ads.ad_users = 0, TRUE, IF(agr_sam_ads.ad_users_reg = 1, IF(agr_sam_ads.x_ad_users = 1, NOT FIND_IN_SET("admin", agr_sam_ads.x_view_users), TRUE) 
     AND 
      IF(agr_sam_ads.ad_users_adv = 1, (agr_sam_ads.adv_nick <> "admin"), TRUE), FALSE))) 
     AND 
      (
       (
        agr_sam_ads.view_type = 1 
       ) 
      OR 
       (
        agr_sam_ads.view_type = 0 
       AND 
        (
         agr_sam_ads.view_pages+0 & 256 
        ) 
       ) 
      ) 
     AND 
      (
       agr_sam_ads.ad_cats = 0 
      ) 
     AND 
      (
       agr_sam_ads.ad_authors = 0 
      ) 
     AND 
      IF(agr_sam_ads.ad_schedule, CURDATE() BETWEEN agr_sam_ads.ad_start_date AND agr_sam_ads.ad_end_date, TRUE) 
     AND 
      IF(agr_sam_ads.limit_hits, agr_sam_ads.hits_limit > agr_sam_ads.ad_hits, TRUE) 
     AND 
      IF(agr_sam_ads.limit_clicks, agr_sam_ads.clicks_limit > agr_sam_ads.ad_clicks, TRUE) 
     AND 
      IF(agr_sam_ads.ad_weight > 0, (agr_sam_ads.ad_weight_hits*10/(agr_sam_ads.ad_weight*1000)) < 1, FALSE) 
    ) AS ad_full_count 
FROM 
    agr_sam_places 
WHERE 
    agr_sam_places.id = 10 
AND 
    agr_sam_places.trash IS FALSE; 

Answer 1

應讀作：

IF (
    expression, 
    valueIfExpressionIsTrue, 
    valueIfExpressionIsValue 
) 

所以把你的代碼的一部分：

IF(
    agr_sam_ads.ad_users = 0, 
    TRUE, 
    IF(
     agr_sam_ads.ad_users_reg = 1, 
     2, 
     3 
    ) 
) 

我看到你熟悉PHP，所以本質上如果這是PHP，代碼將是

if ($arg_sam_ads_ad_users == 0) { 
    return true; 
} else { 
    if ($arg_sam_ads_ad_users_reg == 1) { 
     return 2; 
    } else { 
     return 3; 
    } 
} 

Answer 2

這基本上是在不使用IF _ THEN _ ELSE _ ENDIF語法的情況下創建IF-ELSE樹。

看看下面的例子：

IF(1=0, 1, IF(1=1, 1, 0)) 

這是一個用C語法如下等效：

IF (1=0) { 
    1 
} 
ELSE IF (1=1) { 
    1 
} 
ELSE { 
    0 
} 

比較常見的SQL語法如下：

IF 1=0 THEN 
    1; 
ELSEIF 1=1 THEN 
    1; 
ELSE 
    0; 
ENDIF;