How to interpret financial statement

how to interpret financial statement

How to interpret correlated subqueries?

EDIT: Both answers are great but I think I must clarify my question.

NOTE: I must indicate that in the following, by executed. I mean logically. I understand that the actual execution made by the particular DBMS may use a completely different approach. So what I will ask is not about the actual execution but rather about the evaluation logic of a correlated subquery.

What I particularly want to know is:

"Why a correlated subquery is executed in this way?" For example, for the example query I have given, why isn't it executed in the following way:

"Return the rows of the cities_stores relation whose store_type is equal to some row 's store_type in stores relation. If the resulting relation has at least 1 element, the value of EXISTS clause is TRUE. Otherwise, its value is FALSE ."

but rather in this way:

"For each row processed in the outer query (let's call this row as "current row"), process the inner (correlated) subquery, using the current row's values for the attributes not present in the subquery's FROM clause."

and from where can I learn to think about correlated subqueries in the latter method, instead of the former one?

I hope this is not an unwise question to ask but exactly this is what I don't understand.

Correlated subqueries are highly confusing me.

Example query from here :

Initially, I thought that EXISTS clause would have a value that doesn't change for each row of the outer query and that value would be calculated in the following way:

"Return the rows of the cities_stores relation whose store_type is equal to some row 's store_type in stores relation. If the resulting relation has

at least 1 element, the value of EXISTS clause is TRUE. Otherwise, its value is FALSE ."

Had the EXISTS statement evaluated in this way, it would have a value that does not change among various rows for the outer relation and hence, the query would be equivalent to the following:

And of course this made me think how is an EXISTS statement is useful at all ?

But apparently, that isn't the way a correlated subquery is executed. Apparently, a correlated subquery is executed in the following way:

"For each row processed in the outer query (let's call this row as "current row"), process the inner (correlated) subquery, using the current row's values for the attributes not present in the subquery's FROM clause."

So finally, what I don't understand:

Why is this the way it happens? I mean, why not it doesn't happen in the way I initially thought? (That is: Why it doesn't return the rows of the cities_stores relation whose store_type is equal to some row 's store_type in stores relation?) From exactly where am I supposed to interpret that it is how SQL works?

In a correlated subquery, the subquery is evaluated once for each row processed by the outer query.

But most curiously, a statement similar to this does not exist in MySQL's page about correlated subqueries. So:

Is that an incompleteness in MySQL manual, or is such a statement simply not required? If it is not required, from where am I supposed to understand that it the way how a correlated subquery is evaluated? Or. as another possibility, is the quote I have given simply a vague statement that is not completely correct? If so, how am I supposed to interpret a correlated subquery?

Source: dba.stackexchange.com

Category: Forex

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