Asking about the most simplest SQL technique

Question

select table1.field1,table1.field2
from table1, table2
where table2.category = 'A' and
table1.pk = table2.fk
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If table1 contains 500 records while table2 contain 400,000 records, then table2 should be placed to the most righthand side ?
If table2 has 1000 category 'A' record while
table2 has 80,000 records contain the pk of table1, then table1.pk = table2.fk should be placed on the most lower part in the where clause ?

Answer 1

It used to be the case that you had to consider these issues with some DBMS's, but generally not any more.
Sophisticated systems like Oracle can maintain sets of statistics on tables, and on the distribution of values within fields. If the DBA chooses to, he can run commands to make Oracle analyze individual tables and columns, and the statistics produced will be considered when choosing an access path for the data.
So, the database will know how many records are in the tables in your example query, and roughly how many category 'A' records are in table2 etc.. It will also know whether there are indexes on the columns, and will work out roughly how many bytes must be read to return the result depending on whether the indexes are used, which table is scanned first, and what join technique should be used.
Look in the Oracle documentation for the description of the ANALYZE command, the SQL*Plus command "set autotrace traceonly explain", and in the section on performance have a read of everything to do with the optimizer. It's an important area to know more about if you want to get the most out of Oracle, and is quite an eye-opener if you are moving to Oracle (or any complex RDBMS) frm simpler products (Foxpro, Access etc). Good luck.