SQL連接查詢重構

Question

我正在嘗試基於特定列聯接3個表。 data_customer（主表），data_customer_Address（Cust_id作為外鍵）和Data_customer_contacts（Cust_id作為外鍵）我已經成功實現了以下查詢的結果：

SELECT cst.cust_companyName AS 'Company Name',
       cnt.cntct_fname AS 'Contact First Name',
       cnt.cntct_lName AS 'Contact Last Name',
       cnt.cntct_mainPhone AS 'Main Phn Number',
       cnt.cntct_Mobile AS 'Mobile Number',
       cst.cust_mainEmail AS 'Main Email',
       cnt.cntct_email AS 'Contact Email',
       adr.addressLine1 AS 'Adress line 1',
       adr.addressLine2 AS 'Address Line 2',
       adr.City AS 'City',
       adr.State AS 'State',
       adr.pinZip AS 'Pin/Zip Code'
FROM data_customer AS cst,
     data_customer_Address AS adr,
     data_customer_contacts AS cnt
WHERE cst.[cust_id]='2015Q4'
  AND adr.[cust_id] ='2015Q4'
  AND cnt.[cust_id]='2015Q4';

但是，Cust_id將動態傳遞到查詢中。 如果我不會在任何一個地方傳遞cust_id，我將獲得笛卡爾積。 我嘗試了其他方法，但無法縮短查詢。

請暗示我有什么方法可以改善查詢或改善效能？

注意*：我在Windows上使用Sqlite。

Answer 1

    select cst.cust_companyName as 'Company Name', 
           cnt.cntct_fname as 'Contact First Name', 
           cnt.cntct_lName as 'Contact Last Name', 
           cnt.cntct_mainPhone as 'Main Phn Number',
           cnt.cntct_Mobile as 'Mobile Number', 
           cst.cust_mainEmail as 'Main Email', 
           cnt.cntct_email as 'Contact Email',
           adr.addressLine1 as 'Adress line 1',
           adr.addressLine2 as 'Address Line 2',
           adr.City as 'City', adr.State as 'State', 
           adr.pinZip as 'Pin/Zip Code'
    from data_customer as cst,
         data_customer_Address as adr,
         data_customer_contacts as cnt 
    where 
    cst.[cust_id]=adr.[cust_id]  and cst.[cust_id]=cnt.[cust_id] 
    and cst.[cust_id]='2015Q4';

sql解析通常會首先通過索引（如果有）或完全掃描來獲取表cst中的記錄，然后運行兩個for ... loop進行內部連接表adr和cnt。在這兩個表中都...循環中，表cst將是“外部表”。

Answer 2

您需要學習正確的join語法。 這樣，您的查詢就不會出現此類問題。

簡單規則： 請勿在FROM子句中使用逗號。 始終使用顯式JOIN 。 因此，您的QUERY應該看起來像：

SELECT . . 
FROM data_customer cst JOIN
     data_customer_Address adr
     ON cst.cust_id = adr.cust_id JOIN
     data_customer_contacts cnt
     ON cst.cust_id = cnt.cust_id
WHERE cst.cust_id = '2015Q4';

瞧瞧 省略參數將永遠不會得到笛卡爾積。 而你只需要一次指定客戶ID。

Answer 3

沒關系，我明白了。

select cst.cust_companyName as 'Company Name', 
           cnt.cntct_fname as 'Contact First Name', 
           cnt.cntct_lName as 'Contact Last Name', 
           cnt.cntct_mainPhone as 'Main Phn Number',
           cnt.cntct_Mobile as 'Mobile Number', 
           cst.cust_mainEmail as 'Main Email', 
           cnt.cntct_email as 'Contact Email',
           adr.addressLine1 as 'Adress line 1',
           adr.addressLine2 as 'Address Line 2',
           adr.City as 'City', adr.State as 'State', 
           adr.pinZip as 'Pin/Zip Code'
    from data_customer as cst,
         data_customer_Address as adr,
         data_customer_contacts as cnt 
    where 
    cst.[cust_id]=adr.[cust_id]  and cst.[cust_id]=cnt.[cust_id]    
and 
cst.[cust_id]='2015Q4'

如果您認為還有另一種改進的方法，請告訴我。