The logic behind subqueries/nested query

Question

I am working on some complicated subqueries in Oracle 10g. I read all the documentation, I have reference books and files, and...google. I was hoping, someone might be able to provide a thorough explanation in their own words.
This is my understanding of the matter.
SELECT column1, column2
FROM table1, table2
WHERE join conditions
AND search field1 =
(SELECT column3, column4
FROM table3, table4
WHERE search and join condition0
Now, regarding AND search field1 =, must it be the same as column3 in the sub? How do I know which condition should be set aside for = (Subquery)?
thank you

Answer 1

Hi,
Sorry, I don't understand your question either.
There are a couple of things that I think I can explain for you, however.
First, you should never write, let alone post, unfomatted code. Indent you code so that how it looks on the screen somehow reflects what it does.
When posting code, or any formatted text, on this site, type these 6 characters:
{code}
(small letters only, inside curly brackets) before and after each section of formatted text, to preserve spacing.
If you have to use pseudo-code, make it clear.
For example:
SELECT  column1
,     column2
FROM      table1
,     table2
WHERE      ...     -- join conditions go here
AND     search_field1 = ( SELECT column3
                       ,      column4
                 FROM      table3
                 ,      table4
                 WHERE      ...     -- search and join condition0 goes hereThis can help you spot many erriors. For example, there is no ')' at the end of the sub-query.
The are several different kinds of sub-queries.
One type is called Scalar Sub-Query : it can be used almost everywhere an expression is used.
For example, consider the following query, that finds people whose salary is lower than a given amount:
SELECT  *
FROM    scott.emp
WHERE   sal     < 1250
;The literal 1250 above is an example of an expression. Instead of a numeric literal, we could use a column there:
WHERE   sal     < commor a function:
WHERE   sal     < SQRT (1500000)or a CASE expression, or some combination of them.
What we can't use to the lright of the < sign is anything that is not a SQL datatype (a boolean condition, for example) or any kind of collection that has multiple values.
We could use a scalar sub-query, which is a query that returns no more than one row, and exactly one column.
For example
SELECT  *
FROM    scott.emp
WHERE   sal     < ( SELECT  AVG (sal)
              FROM    scott.emp
;You will get a compile error (ORA-00913) if the sub-query has more than one column, and a run-time error (ORA-01427) if it returns more than one row.
Scalar sub-queries can be correlated to the parent query.
The example above picked employees who earn less than the average of all employees.
The example below picks employees who earn less that the average for their job:
SELECT  *
FROM    scott.emp     m     -- m for Main
WHERE   sal     < ( SELECT  AVG (sal)
              FROM    scott.emp
              WHERE   job     = m.job
;