MySQL優化LEFT Join以使用同一表進行大數據查詢

Question

我在這里有一個表SQL

| id | dataid |  datarow | datacolumn | datavalue
_________________________________________________
|  1  |  1     |     1    | FirstName   |  John
|  1  |  1     |     1    | LastName    |  Lobo
|  1  |  1     |     1    | Age         |  35
|  1  |  1     |     2    | FirstName   |  Mich
|  1  |  1     |     2    | LastName    |  Handness
|  1  |  1     |     2    | Age         |  22
|  1  |  1     |     3    | FirstName   |  Mike
|  1  |  1     |     3    | LastName    |  Longbow
|  1  |  1     |     3    | Age         |  55

它將輸出類似這樣的內容

FirstName  LastName Age
   John      Lobo   35
   Mich      Handness 22
   Mike      Longbow  55
SELECT DISTINCT     
        t1.datavalue AS Firstname, 
        t2.datavalue AS Lastname,
        t3.datavalue AS Age

FROM largedatatable t1 
        LEFT JOIN (SELECT datavalue,datarow FROM largedatatable WHERE           datacolumn='LastName') t2 ON t1.datarow = t2.datarow
        LEFT JOIN (SELECT datavalue,datarow FROM largedatatable WHERE datacolumn='Age') t3 ON t1.datarow = t3.datarow

WHERE t1.dataid = 1 

這給我的結果像

Firstname  LastName   Age
John        Lobo      35
Mich      Handness    22
Mike      Longbow     55

現在，腳本可以正常運行了。 但是如果查詢的數據量很大，則查詢速度很慢。 無論如何，我仍然可以優化此SQL查詢。 謝謝！

Answer 1

我認為這可以解決您的問題，或者至少可以更快地運行，因為它將只提取一次表：

select 
    max(case datacolumn when 'FirstName' then datavalue else '' end) as FirstName,
    max(case datacolumn when 'LastName' then datavalue else '' end) as LastName,
    max(case datacolumn when 'Age' then datavalue else '' end) as Age
from 
    large
group by datarow

在小提琴這里查看： http ://sqlfiddle.com/#!2/ f4045/11

Answer 2

我會確保您在（datarow，datacolumn）上有一個索引來幫助您進行連接，但是您的連接可以簡化為

SELECT 
        t1.datavalue AS Firstname, 
        t2.datavalue AS Lastname,
        t3.datavalue AS Age
   FROM 
      largedatatable t1
         LEFT JOIN largedatatable t2
            on t1.datarow = t2.datarow
           and t2.datacolumn = 'LastName'
         LEFT JOIN largedatatable t3
            on t1.datarow = t3.datarow
           and t3.datacolumn = 'Age'
   WHERE
      t1.datacolumn = 'FirstName'

僅基於名字列，這將僅通過您的主表一次，但是不需要第二個實例的列，而無需聚合的MAX（）或GROUP BY。