From (SQL)

From is a fundamental keyword in Structured Query Language (SQL) used to specify the table or tables from which data is to be retrieved in a SELECT statement. It is a mandatory clause in most SELECT queries, defining the data source for the operation.

The From clause identifies the base table(s), view(s), or derived table(s) that contain the columns being selected and upon which any filtering, joining, or grouping operations will be performed. It can also be used with subqueries, treating the subquery as a derived table.

When joining data from multiple tables, the From clause lists all involved tables, typically used in conjunction with a JOIN clause (e.g., INNER JOIN, LEFT JOIN, RIGHT JOIN) to specify the relationship between them. The order of tables listed in the From clause can sometimes impact performance, depending on the specific database system and query optimizer.

The From clause is parsed early in the SQL query execution process, allowing the database engine to locate and access the specified data sources before applying other clauses like WHERE, GROUP BY, or HAVING. Without a From clause, the SELECT statement would not know where to fetch the requested data.