From (SQL) Examples and Requirements

[sukishan]

The SQL From clause is the source of a rowset to be operated upon in a Data Manipulation Language (DML) statement. From clauses are very common, and will provide the rowset to be exposed through a Select statement, the source of values in an Update statement, and the target rows to be deleted in a Delete statement.

FROM is an SQL reserved word in the SQL standard

The FROM clause is used in conjunction with SQL statements, and takes the following general form:

SQL-DML-Statement
FROM table_name
WHERE predicate

The From clause can generally be anything that returns a rowset, a table, view, function, or system-provided information like the Information Schema, which is typically running proprietary commands and returning the information in a table form.

Examples
The following query returns only those rows from table mytable where the value in column mycol is greater than 100.

SELECT *
FROM   mytable
WHERE  mycol > 100

Requirement
The From clause is technically required in relational algebra and in most scenarios to be useful. However many relational dbms implementations may not require it for selecting a single value, or single row.

SELECT 3.14 AS Pi
Other systems will require a From statement with a keyword, even to select system data.

SELECT to_char(sysdate, 'Dy DD-Mon-YYYY HH24:MI:SS') AS "Current Time"
FROM dual;