MySQL 子查詢 練習題

測試數據表：

suppliers 表

  s_id  s_name          s_city     s_zip   s_call  
------  --------------  ---------  ------  --------
   101  FastFruit Inc.  Tianjin    300000  48075   
   102  LT Supplies     Chongqing  400000  44333   
   103  ACME            Shanghai   200000  90046   
   104  FNK Inc.        Zhongshan  528437  11111   
   105  Good Set        Taiyuang   030000  22222   
   106  Just Eat Ours   Beijing    010     45678   
   107  DK Inc.         Zhengzhou  450000  33332   

fruits 表

f_id      s_id  f_name      f_price  
------  ------  ----------  ---------
a1         101  apple       5.20     
a2         103  apricot     2.20     
b1         101  blackberry  10.20    
b2         104  berry       7.60     
b5         107  xxxx        3.60     
bs1        102  orange      11.20    
bs2        105  melon       8.20     
c0         101  cherry      3.20     
l2         104  lemon       6.40     
m1         106  mango       15.60    
m2         105  xbabay      2.60     
m3         105  xxtt        11.60    
o2         103  coconut     9.20     
t1         102  banana      10.30    
t2         102  grape       5.30     
t4         107  xbababa     3.60   

第一條語句，不太懂，到底是返回了查詢結果的信息表，還是 ture
（推測點：SELECT * FROM suppliers WHERE fruits.s_id = suppliers.s_id AND fruits.s_id=107` 語句中，fruits與suppliers形成了 內連接）

SELECT * FROM fruits
WHERE EXISTS
(SELECT * FROM suppliers 
WHERE fruits.`s_id` = suppliers.`s_id` AND  fruits.s_id=107);

f_id      s_id  f_name   f_price  
------  ------  -------  ---------
b5         107  xxxx     3.60     
t4         107  xbababa  3.60     

下面爲 兩個表內連接 的查詢結果 ：

SELECT * FROM suppliers ,fruits
WHERE fruits.`s_id` = suppliers.`s_id` AND  fruits.s_id=107

  s_id  s_name   s_city     s_zip   s_call  f_id      s_id  f_name   f_price  
------  -------  ---------  ------  ------  ------  ------  -------  ---------
   107  DK Inc.  Zhengzhou  450000  33332   b5         107  xxxx     3.60     
   107  DK Inc.  Zhengzhou  450000  33332   t4         107  xbababa  3.60     

第二條語句，查詢之後返回ture，然後查詢出了 fruits表的全部數據
（推測點： SELECT * FROM suppliers WHERE s_id=107 語句中 suppliers表與上層的select中 fruits沒有連接條件）

SELECT * FROM fruits
WHERE EXISTS
(SELECT * FROM suppliers WHERE s_id=107);

f_id      s_id  f_name      f_price  
------  ------  ----------  ---------
a1         101  apple       5.20     
a2         103  apricot     2.20     
b1         101  blackberry  10.20    
b2         104  berry       7.60     
b5         107  xxxx        3.60     
bs1        102  orange      11.20    
bs2        105  melon       8.20     
c0         101  cherry      3.20     
l2         104  lemon       6.40     
m1         106  mango       15.60    
m2         105  xbabay      2.60     
m3         105  xxtt        11.60    
o2         103  coconut     9.20     
t1         102  banana      10.30    
t2         102  grape       5.30     
t4         107  xbababa     3.60