Subquery Nested by FROM

Function

This statement is used to nest subquery by FROM and use the subquery results as the data source of the external SELECT statement.

Syntax

1

SELECT [ALL | DISTINCT] attr_expr_list FROM (sub_query) [alias];

Keyword

• All is used to return repeated rows. By default, all repeated rows are returned. It is followed by asterisks (*) only. Otherwise, an error will occur.
• DISTINCT is used to remove the repeated line from the result.

Precautions

• The to-be-queried table must exist. If this statement is used to query a table that does not exist, an error is reported.
• The subquery nested in FROM must have an alias. The alias must be specified before the running of the statement. Otherwise, an error is reported. It is advised to specify a unique alias.
• The subquery results sequent to FROM must be followed by the specified alias. Otherwise, an error is reported.

Example

To return the names of students who select the courses in the course_info table and remove the repeated records using DISTINCT, run the following statement:

1
2

SELECT DISTINCT name FROM (SELECT name FROM student_info
  JOIN course_info ON student_info.courseId = course_info.courseId) temp;