The document provides information on SQL subqueries in Oracle 11g, detailing their syntax and types, including single-row and multiple-row subqueries. It explains the execution order, guidelines for use, and provides examples for clarity. Additionally, it highlights common issues and illegal queries related to subquery usage.

1. SQL Fundamentals Oracle 11g
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Lecture#8
SQL Subqueries

2. Subqueries
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3. Subqueries(cont’d)
Syntax for subqueries:
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4. Subqueries(cont’d)
•The subquery(inner query) is executed once before the
main query.
•The result of subquery is used by the main query(outer
query).
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5. Subqueries(cont’d)
There are following types of subqueries:
•Single row
•Multiple row
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6. Subqueries(cont’d)
Guidelines for using subqueries:
•Enclose subquery in parenthesis
•Place subquery on the right side of comparison condition
•Use single row operators with single row queries and
multiple with multiple queries
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7. Subqueries(cont’d)
There are two types of subqueries:
•Single row subqueries
•Multiple row subqueries
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8. Subqueries(cont’d)
Single row subqueries:
•Returns only one row
•Use single row comparison operators i.e. =, < , > , <= ,>=, <>
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9. Subqueries(cont’d)
Single row subqueries:
•Example:
SELECT *
FROM teacher
WHERE dept_id =
(SELECT dept_id
FROM department
WHERE dept_name = ‘UIIT’);
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10. Subqueries(cont’d)
More than one single row subqueries:
•Example:
SELECT *
FROM teacher
WHERE dept_id =
(SELECT dept_id
FROM department
WHERE dept_name = ‘UIIT’)
AND salary >
(SELECT AVG(salary)
FROM teacher) ;
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11. Subqueries(cont’d)
HAVING clause single row subqueries:
•Example:
SELECT dept_id, MIN(salary)
FROM employee
GROUP BY dept_id
HAVING MIN(salary) >
(SELECT MIN(salary)
FROM employee
WHERE dept_name = ‘UIIT’);
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12. Subqueries(cont’d)
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Illegal queries are as following:
• You cannot use multiple rows return queries for single row
operators

13. Subqueries(cont’d)
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Empty result subqueries are as following:
• ‘no rows selected’ because subquery returns nothing

14. Multiple-Row Subqueries(cont’d)
•Return more than one row
•Uses multiple row comparison operators i.e.
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15. Multiple-Row Subqueries(cont’d)
Use of IN operator in multiple-row subqueries
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16. Multiple-Row Subqueries(cont’d)
Use of ANY operator in multiple-row subqueries:
• ‘<ANY’ means less than maximum of returned values
• ‘>ANY’ means more than the minimum of returned values
•‘=ANY’ is equivalent to ‘IN’ operator
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17. Multiple-Row Subqueries(cont’d)
Use of ALL operator in multiple-row subqueries:
• ‘<ALL’ means less than maximum of returned values
• ‘>ALL’ means more than the minimum of returned values
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18. Multiple-Row Subqueries(cont’d)
NULL values in multiple-row subqueries:
• If one of the returned values by inner query is NULL value then the entire query returns no
values
•‘NOT IN’ operator is equivalent to ‘<>ALL’
•This issue can be resolved by using an additional WHERE clause in the inner query i.e WHERE
mgr.manager_id IS NOT NULL
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