SQL 1.60 ALL Operator

Understanding the `ALL` Operator

SQL (Structured Query Language) is the standard language for managing and manipulating databases. This tutorial will focus on using the ALL operator in SQL. The ALL operator is used to compare a value to all values in another value set or result from a subquery.

1. Understanding the `ALL` Operator

The ALL operator allows you to compare a value to all values returned by a subquery. It returns TRUE if the comparison is TRUE for every value in the subquery result.

2. Using `ALL` with SELECT Statements

Syntax

SELECT column_name(s)
FROM table_name
WHERE column_name operator ALL (subquery);

column_name(s): The column or columns you want to retrieve.
table_name: The table from which you want to retrieve the data.
operator: A comparison operator like =, !=, >, >=, <, or <=.
subquery: A subquery that returns a set of values.

3. Using `ALL` with Comparison Operators

Scenario 1: Fetching records where a column value is greater than all values returned by a subquery

Suppose we have two tables, students and courses.

Table: students

student_id	name	age
1	Alice	22
2	Bob	23
3	Charlie	21

Table: courses

course_id	student_id	grade
101	1	88
102	2	92
103	3	85
104	1	90
105	2	87

Objective: Find students whose age is greater than the age of all students in a specific set of courses.

Step-by-Step Example

Subquery to get ages of students enrolled in course 101 and 102: SELECT age FROM students WHERE student_id IN (SELECT student_id FROM courses WHERE course_id IN (101, 102));
Main query to find students older than all ages from the subquery:
sql SELECT name FROM students WHERE age > ALL (SELECT age FROM students WHERE student_id IN (SELECT student_id FROM courses WHERE course_id IN (101, 102)));

4. Practical Examples

Example 1: Retrieve students with grades higher than all grades in a specific course

Table: students

student_id	name	age
1	Alice	22
2	Bob	23
3	Charlie	21

Table: courses

course_id	student_id	grade
101	1	88
102	2	92
103	3	85
104	1	90
105	2	87

Objective: Find the names of students who have a grade higher than all grades of students in course 103.

Step-by-Step Example

Subquery to get grades of students in course 103: SELECT grade FROM courses WHERE course_id = 103;
Main query to find students with grades higher than all grades in course 103:
sql SELECT name FROM students WHERE student_id IN (SELECT student_id FROM courses WHERE grade > ALL (SELECT grade FROM courses WHERE course_id = 103));

Explanation

Subquery: SELECT grade FROM courses WHERE course_id = 103;
This fetches grades of students enrolled in course 103 (e.g., returns 85).
Main query:

  SELECT name 
  FROM students 
  WHERE student_id IN (SELECT student_id 
                       FROM courses 
                       WHERE grade > ALL (SELECT grade 
                                          FROM courses 
                                          WHERE course_id = 103));

This fetches names of students whose grades are higher than all grades returned by the subquery (i.e., higher than 85).

5. Summary

The ALL operator is used to compare a value to all values in a subquery.
It returns TRUE if the condition is TRUE for every value in the subquery result.
Common use cases include finding records that meet a criterion compared to a set of values from another query.

Sample Outputs for Different Scenarios

Finding students older than all students in courses 101 and 102:

If subquery returns ages 22 and 23, the main query might return:
plaintext No students found, as no student is older than 23.

Finding students with grades higher than all grades in course 103:

If subquery returns grade 85, the main query might return:
plaintext Bob

By following this tutorial, you should have a solid understanding of using the ALL operator in SQL and be able to apply it to various scenarios in database queries.

Understanding the `ALL` Operator