Question

下面是我的选择查询

#include <stdio.h>
#include <stdlib.h>

int main(){
    char **p;

    p = (char**) malloc (2*sizeof(char*));
    *p = (char*) malloc (2*sizeof(char));

    **p = 'k';
    **(p+1) = 'f';
    *p = *p+1;
    **p = 'c';
    **(p+1) = 'd';
}

我想基于customerid的count（*）值更新表“ custtable”中的“级别”和“折扣”列

如果count（*）<2，则级别= 1，折扣= 10
如果count（*）> 3，则级别= 3，折扣= 20
如果为0，则均为0

如何在Mysql中执行此操作？

Answer 1

由于您没有提供测试用例，所以我自己做了。可能并不完美，但是比 none 更好。

SQL> create table custtable (customerid number, c_level number, discount number);

Table created.

SQL> insert into custtable
  2    select 1, null, null from dual union all
  3    select 2, null, null from dual;

2 rows created.

SQL>
SQL> create table orders (customerid number, orderno number);

Table created.

SQL> insert into orders
  2    select 1, 100 from dual union all
  3    select 3, 300 from dual;

2 rows created.

SQL>
SQL> create table orderitem (orderno number);

Table created.

SQL> insert into orderitem
  2    select 100 from dual union all
  3    select 300 from dual;

2 rows created.

这是您的查询：

SQL> select d.customerid, count(*) as count
  2    from orderitem i join orders d on d.orderno = i.orderno
  3    group by d.customerid;

CUSTOMERID      COUNT
---------- ----------
         1          1
         3          1

SQL>

为了执行 update ，我建议使用MERGE语句，例如

SQL> merge into custtable t
  2    using (select d.customerid, count(*) as cnt
  3           from orderitem i join orders d on d.orderno = i.orderno
  4           group by d.customerid
  5          ) x
  6    on (t.customerid = x.customerid)
  7  when matched then update set
  8    t.c_level = case when x.cnt < 2 then 1
  9                     when x.cnt > 3 then 3
 10                     when x.cnt = 0 then 0
 11                end,
 12    t.discount = case when x.cnt < 2 then 10
 13                      when x.cnt > 3 then 20
 14                      when x.cnt = 0 then 0
 15                 end;

1 row merged.

结果：

SQL> select * From custtable;

CUSTOMERID    C_LEVEL   DISCOUNT
---------- ---------- ----------
         1          1         10
         2

SQL>

Answer 2

您可以在UPDATE语句中使用相关子查询来做到这一点：

update custtable
    set (level, discount) = 
         (select (case when count(*) = 0 then 0
                       when count(*) <= 2 then 1
                       else 3
                  end) as level,
                 (case when count(*) = 0 then 0
                       when count(*) <= 2 then 10
                       else 20
                  end) as discount                  
          from Orderitem oi join
               orders o
               on oi.orderno = o.orderno 
          where o.customerid = custtable.customerId
         );

请注意，Oracle使您可以同时在update中更新多个列。

我也略微更改了逻辑，因此包括了“ 2”的计数。

根据来自另一个表的count（*）值更新表中的多列

2 个答案: