A Basic Introduction To SQL

The most basic syntax of SQL starts with a SELECT clause and a FROM clause. A SELECT clause tells the database WHAT you want to select.
A FROM clause tells the database where to find that data.From SQL*Plus, login as whatever user you have available and enter:SELECT table_name FROM all_tables;When I run my query,

I get a list of tables followed by the text "113 rows selected.". The SELECT clause is TABLE_NAME and the FROM clause is all_tables. The semi colon at the end tells Oracle that

you have completed your command. You MUST enter a semi-colon or a slash before Oracle will process you command.This was a very useful command but it could be made more useful. The list of tables was just that, a list.

If we were looking for a specific table, it would be hard with an unordered list like that. Let's try putting some order to it.Enter:

SELECT table_name FROM all_tables ORDER BY table_name /

Ok, there are three differences here. There is no semi colon but there is a slash, "/". The slash also tells Oracle that Oracle DBA have completed a command. The slash must appear ion column 1 on a line by itself. A semi colon can follow anywhere as long as there are no blank lines between it and the command.Type slash on a line by itself and Oracle will re-execute the previous command. Try it now.Ok,

we also added an ORDER BY clause to our command. The ORDER BY tells Oracle to, oddly enough, order the result set. What did it order by? TABLE_NAME.You can scroll through the list and find a specific table. Scroll through it now and find the table, DUAL. If you don't see this table, something is either not configured properly in your database or not configured properly with your user. DUAL is a special table provide by Oracle. It should always have one row and only have one column. That column, DUMMY, should always have the value 'X'.

Enter: SELECT table_name FROM all_tables WHERE table_name = 'DUAL' /


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