如何避免关联CASE中的子查询？

Question

假设我们有以下四个表：

create table table_x(x number, title varchar2(100)); 

create table table_y(y number, x number, title varchar2(100)); 

create table table_z(z number, y number, x number, title varchar2(100)); 

create table table_main(z number, y number, x number); 

它需要根据此规则在table_main每一行获取title：

select 
    case 
     when z is not null 
     then (select title from table_z where x = tm.x and y = tm.y and z = tm.z) 

     when y is not null 
     then (select title from table_y where x = tm.x and y = tm.y) 

     when x is not null 
     then (select title from table_x where x = tm.x) 
    end as title 
from table_main tm; 

我在寻找生产相同的结果更可读的解决方案。

有没有一种方法来重构查询替换case when语句与表连接或以某种方式？

Answer 1

我的做法是：

SELECT COALESCE(tz.title, ty.title, tx.title) AS mytitle 
    FROM table_main tm 
    LEFT OUTER JOIN table_x tx 
    ON tx.x_number = tm.x_number 
    LEFT OUTER JOIN table_y ty 
    ON ty.x_number = tm.x_number 
    AND ty.y_number = tm.y_number 
    LEFT OUTER JOIN table_z tz 
    ON tz.x_number = tm.x_number 
    AND tz.y_number = tm.y_number 
    AND tz.z_number = tm.z_number