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 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.[unreliable source]
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
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 - known as DUAL table in Oracle database.
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;
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