Question

I have an SQL query where I want to use one of the column value of the main query in the sub query.

The query is:

select **tool.item**, asset.id, tool.date,
       (select freq from workorder
        where type = 'CP' and itemnum = **tool.item**) freq, asset.pm
from tool,
     asset
where too.num = asset.num 
  and asset.status = 'ACTIVE';

In this query I want use the fetched tool.item value in the sub query.

item assetid date       pm   freq

A1    1      12-NOV-15  123  freq from workorder where itemnum ='A1'
A2    2      13-NOV-15  124  freq from workorder where itemnum ='A2'

Could you help me with this? Thanks in advance.

Answer 1

I strongly encourage you to do two things:

Learn proper JOIN syntax (never use commas in the from clause.
Use abbreviations for table aliases.

So, write the query as:

select t.item, a.id, t.date,
       (select wo.freq
        from workorder wo
        where wo.type = 'CP' and wo.itemnum = t.item
       ) as freq, 
       a.pm
from tool t join
     asset a
     on t.num = a.num 
where a.status = 'ACTIVE';

A correlated subquery is a query where the subquery uses columns from the outer query. In this case, the correlation uses t.item in the where clause. When using correlated subqueries I very, very, very strongly recommend that you always use table aliases. It is very easy to make mistakes with column names, and these problems can be quite hard to find.

Answer 2

it's similar to normal join, you need join you subquery in column with your tables in from section if the query returns null or 1 value it works ok if it returns more than 1 value you will have exception

select tool.item, asset.id, tool.date,
       (select freq from workorder
        where type = 'CP' and itemnum = tool.item) freq, asset.pm
from tool,
     asset
where tool.num = asset.num 
  and asset.status = 'ACTIVE';

How to use main query column value in sub-query?

2 个答案: