如何在IBM DB2中获得此结果？

Question

例如，我有两个模式：SCHEMA_1和SCHEMA_2。在SHEMA_1我有一个名为表表。该表格包括两个字段：FIELD_1，FIELD_2。在FIELD_1的TABLE中，我有一些字母：A，B，C。FIELD_2具有SCHEMA 2的表名：TABLE_10，TABLE_20，TABLE_30。

SCHEMA_2包括三个表：TABLE_10，TABLE_20，TABLE_30有一些数字。 enter image description here

我必须编写查询来获取SCHEMA_2中每个表的最大数量。我怎样才能得到这样的结果> enter image description here

Answer 1

试试这个：

create table schema1.tableA 
    (field_1 char(1), 
    field_2 varchar(10)); 
insert into schema1.tableA 
    values ('A','table_10'), 
     ('B','table_20'), 
     ('C','table_30'); 

create table schema2.table_10 
    (field_1 dec(5,0)); 
insert into schema2.table_10 
    values (20), (30), (40); 
create table schema2.table_20 
    (field_1 dec(5,0)); 
insert into schema2.table_20 
    values (6), (9), (12); 
create table schema2.table_30 
    (field_1 dec(5,0)); 
insert into schema2.table_30 
    values (10), (15), (20); 

with tmp (table_name, field_1) as (
    select 'table_10', max(field_1) from schema2.table_10 
    union all 
    select 'table_20', max(field_1) from schema2.table_20 
    union all 
    select 'table_30', max(field_1) from schema2.table_30) 
select a.field_1, b.field_1 
    from schema1.tableA a 
    join tmp b on b.table_name = a.field_2; 

如果你有太多的表，以使上述工作，你可以使用一个用户定义的函数是这样的：

create or replace function MaxNbr 
    (p_TableName  varchar(128), 
    p_TableSchema varchar(128)) 
    Returns dec(5,0) 
    language sql 
    not deterministic 
    no external action 
    reads sql data 
    returns null on null input 
    not fenced 
begin 
    declare l_stmt  varchar(1024); 
    declare l_table varchar(128); 
    declare l_schema varchar(128); 
    declare l_result dec(5,0); 

    set l_table = replace(upper(p_TableName),'"',''); 
    set l_schema = replace(upper(p_TableSchema),'"',''); 
    set l_stmt = 'values (select max(field_1) from "' || l_schema || '"."' || 
       trim(l_table) || '") into ?'; 

    prepare S1 from l_stmt; 
    allocate sql descriptor 'D1'; 
    describe S1 using sql descriptor 'D1'; 
    execute S1 into sql descriptor 'D1'; 
    get sql descriptor 'D1' value 1 l_result = data; 
    deallocate sql descriptor 'D1'; 

    return l_result; 
end; 

这是必要的，因为您不能将变量用于标识符，如表名称或模式名称。

要使用用户定义的函数，你可以这样做：

set schema schema1; 
set path = udf_schema; 

select field_1, MaxNbr(field_2, 'schema2') as field_2 
    from tableA;