From (SQL)
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. {{cite web | url=https://www.drupal.org/docs/develop/coding-standards/list-of-sql-reserved-words | title=Reserved Words in SQL}}
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.{{cite web |url=https://docs.microsoft.com/en-us/sql/relational-databases/system-information-schema-views/system-information-schema-views-transact-sql?view=sql-server-2017 |title=System Information Schema Views (Transact-SQL) }}
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 - known as DUAL table in Oracle database.{{cite web |url=https://docs.oracle.com/cd/B19306_01/server.102/b14200/queries009.htm |title=Selecting from the DUAL Table}}
SELECT 3.14 AS Pi
Other systems will require a From statement with a keyword, even to select system data.{{cite web |url=http://infolab.stanford.edu/~ullman/fcdb/oracle/or-time.html |title=Oracle Dates and Times}}
select to_char(sysdate, 'Dy DD-Mon-YYYY HH24:MI:SS') as "Current Time"
from dual;
References
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