Question

我对SQL很陌生，我很难看到我在这个剧本中错过了什么。

每次我尝试运行它时，都会花费很长时间并因内存不足而导致错误。

select r.CONSTITUENT_ID
     , case when r.SEX = '1'then 'Male'
           when r.SEX = '2' then 'Female' else 'Unknown'end as Gender
     , case when r.KEY_INDICATOR = 'O' then 'Organisation'
            when r.KEY_INDICATOR = 'I' then 'Individual'end as record_Classification
     , case when cc.CODE = '1924' then 'Bristol Affiliated'
            when cc.CODE = '1963' then 'Gloucester Affiliated'
            when cc.CODE = '2045' then 'Wiltshire Affiliated'
            else 'n/a' end as DW_Affiliation
from RECORDS r, CONSTITUENT_CODES cc

Answer 1

这个

from RECORDS r,CONSTITUENT_CODES cc`

是交叉连接，意味着show me every row from RECORDS with every row CONSTITUENT_CODES，因此它返回[来自RECORDS的行数] * [来自CONSTITUENT_CODES的行数]行。您必须使用join或where来消除cross join

from RECORDS r,CONSTITUENT_CODES cc
where r.column  = cc.column

或

from RECORDS r
join CONSTITUENT_CODES cc on r.column  = cc.column

我不知道你的桌子结构，但也许这会起作用

select r.CONSTITUENT_ID
     , case when r.SEX = '1'then 'Male'
           when r.SEX = '2' then 'Female' else 'Unknown'end as Gender
     , case when r.KEY_INDICATOR = 'O' then 'Organisation'
            when r.KEY_INDICATOR = 'I' then 'Individual'end as record_Classification
     , case when cc.CODE = '1924' then 'Bristol Affiliated'
            when cc.CODE = '1963' then 'Gloucester Affiliated'
            when cc.CODE = '2045' then 'Wiltshire Affiliated'
            else 'n/a' end as DW_Affiliation
from RECORDS r, CONSTITUENT_CODES cc
where r.CONSTITUENT_ID = cc.CONSTITUENT_ID

使用推荐的连接样式，它看起来像

select r.CONSTITUENT_ID
     , case when r.SEX = '1'then 'Male'
           when r.SEX = '2' then 'Female' else 'Unknown'end as Gender
     , case when r.KEY_INDICATOR = 'O' then 'Organisation'
            when r.KEY_INDICATOR = 'I' then 'Individual'end as record_Classification
     , case when cc.CODE = '1924' then 'Bristol Affiliated'
            when cc.CODE = '1963' then 'Gloucester Affiliated'
            when cc.CODE = '2045' then 'Wiltshire Affiliated'
            else 'n/a' end as DW_Affiliation
from RECORDS r 
join CONSTITUENT_CODES cc on r.CONSTITUENT_ID = cc.CONSTITUENT_ID

我在RECORDS CONSTITUENT_ID中看到此列可用于加入来自CONSTITUENT_CODES的列ID - 我猜CONSTITUENT_ID

我的SQL脚本中缺少什么？

1 个答案: