查询数据取决于其他表中的数据

Question

I'm trying to get data from a table which is only related through two different tables. For example, I have information "dataA" and need to get information "dataD".

How do I write a query to display dataA and dataD as they relate? I don't want to display all instances of dataD, just the ones that related to dataB and then to dataA. I'm sorry if this doesn't make enough sense.

Answer 1

You'll use JOINs to do this. Specifically, in this case, a LEFT OUTER JOIN is your best bet. Something like:

SELECT
    Table1.dataA,
    Table3.dataD
FROM
    Table1
    LEFT OUTER JOIN Table2 ON Table1.dataB = Table2.dataB
    LEFT OUTER JOIN Table3 ON Table2.dataC = Table3.dataC

Answer 2

Working on the assumption that the "data" entries you put under each table name represent columns in the table, this is fairly simple:

select dataA
  ,    dataD
from   Table1 t1
    join Table2 t2
         on t1.dataB = t2.dataB
    join Table3 t3
         on T2.dataC = t3.dataC

If you need all the dataA regardless of whether there's a matching dataD (or other intervening column) you would change where I wrote 'join' to 'left join' .

And your question makes plenty of sense. :) I was just a little unclear from the diagram what you had in mind.