Attach query or subquery

Question

Are there rules for developers when you need to use a union instead of a subquery, or are they the same.

Answer 1

Depends on the DBMS. You should compare execution plans for both queries.

In my experience with Oracle 10 and 11, execution plans are always the same.

Answer 2

The first principle: "Specify the request correctly." The second principle is that "simply and explicitly indicate the request" (that is where you usually make the choice). The third is "request a request so that it works efficiently."

dbms , , (, , ).

MySQL , . Oracle, SQL Server, Informix dbms, , . MySQL, -, , .

Answer 3

, .

Answer 4

, - , .

, JOIN- ON WHERE ( , ).

Answer 5

.

Answer 6

, . - - ,

, . , - , .

, Alex

Answer 7

( , , " - " ).

, .

Answer 8

SQL Server , , , . , . , . - -.