MySQL查詢 - WHERE子句執行時可能還沒有確定where子句列值的未知列

Question

SELECT area_id, 
      area_name, 
      (select count(*) from applications 
        where claims_status=1 and 
        center_name=c.area_id) as cont 
    FROM apparea c where cont<>0 

我試圖從anothere表中獲取字段和相關計數，但上面的查詢不起作用。查詢涉及兩個不同的表格（apparea，應用程序）。上述查詢有錯誤，我正在尋找替代方法來實現這一點。

Answer 1

您的列cont的別名在WHERE子句中不可用。您將要使用類似這樣的東西：

SELECT area_id, 
    area_name, 
    cont 
FROM 
(
    SELECT area_id, 
    area_name, 
    (select count(*) 
    from applications 
    where claims_status=1 
    and center_name=c.area_id) as cont 
    FROM apparea c 
) c 
where cont<>0 

這也可以寫成使用LEFT JOIN：

select c.area_id, 
    c.area_name, 
    a.cont 
from apparea c 
left join 
(
    select count(*) cont, 
    center_name 
    from applications 
    where claims_status=1 
    group by center_name 
) a 
    on c.area_id = a.center_name 

Answer 2

嘗試此查詢

SELECT 
    c.area_id, 
    c.area_name, 
    cnt 
FROM 
    apparea c, 
    (select 
      center_name, 
      count(*) AS cnt 
    from 
      applications 
    where 
      claims_status=1 
    GROUP BY 
      center_name 
    HAVING 
      count(*) > 0) cont 
where 
    c.area_id = cont.center_name; 

得到了各center_name計數和然後加入表格以獲得每個區域的數量

Answer 3

使用HAVING而不是where。

因爲這是aliases的問題。

It is not permissible to refer to a column alias in a WHERE clause, because the column 
value might not yet be determined when the WHERE clause is executed. 
See Section C.5.5.4, 「Problems with Column Aliases」. 

http://dev.mysql.com/doc/refman/5.0/en/problems-with-alias.html

來源：http://dev.mysql.com/doc/refman/5.0/en/select.html