MANAGING DATA(BASES) USING SQL (NON-PROCEDURAL SQL, …ucb-access.org/Files/SQL/PPPresentation/PPSQLClass7.pdf · 2018-08-17 · 10.2 SUBQUERIES AS COLUMN EXPRESSIONS Must link departments.department_id

MANAGING DATA(BASES) USING SQL

(NON-PROCEDURAL SQL, X401.9)

Professional Program: Data Administration and Management

Instructor: Michael Kremer, Ph.D.Technology & Information Management

Class 7

AGENDA

10. Subqueries

10.1 Overview of Subqueries

10.2 Subqueries as Column Expressions

10.3 Subqueries as Filter Expressions

10.4 Subqueries as Datasource Expressions

10.5 Correlated Subqueries

11. Advanced Subqueries

11.1 Pivot

11.2 Subquery Factoring

11.3 Top-N-Analysis

Subqueries

10.

10.1 OVERVIEW OF SUBQUERIES

❑ Subquery is a query inside a main query (or inner, nested

SELECT, or sub-SELECT)

❑ Subquery can be placed in different

locations of the main query:

❑ Same syntax with a

few exceptions:

❑ Two main types of subqueries,

based on processing order:

❑ Simple or nested subquery is a subquery that is processed completely

before the outer or main query is executed

❑ Linked or correlated is processed for each row of the outer query

❑ More than one level of nesting, innermost query is processed

first → result is used in next query, etc.

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❑ Subqueries are categorized

by the structure of data

returned:

❑ Basic example:

❑ Display all employees

earning more than the

average of all employees

❑ What kind of subquery?

A scalar subquery, one

and only one value returned!

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❑ Row subqueries used in the WHERE clause of the main query

❑ SQL standard defines row value constructor (used in WHERE,

HAVING, ON clause):

❑ In Oracle, only in

conjunction with subqueries:

❑ Table subquery mostly used

in FROM clause:

❑ Scalar subquery returns at

most only one value, it is

most widely used:

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10.2 SUBQUERIES AS COLUMN EXPRESSIONS

❑ Column reference in the SELECT clause can be replaced by a

subquery

❑ Must use scalar subquery in place of a column reference since

column reference returns exactly one value per row of the query

❑ Many options in

SELECT clause:

❑ To guarantee

scalar value:

❑ Aggregate query with no GROUP BY (or WHERE clause to filter)

❑ Regular query with primary key as the filter

❑ Regular query selecting one specific value

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❑ Error message

when not

using a scalar

subquery:

❑ Error message

when selecting

more than one

column in

subquery:

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❑ Back to first

example, now add

same subquery

from WHERE

clause as column

expression:

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❑ Correlated subquery

as column

expression

❑ Link subquery to

main query using

department_id

❑ Result does not

make sense, the

department max

salary is the same

for all departments!

❑ Department_id in WHERE clause is the same on both sides,

subquery is using the department_id from subquery FROM

clause (departments table)

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❑ Must link

departments.department_id

to employees.department_id

❑ Use table aliases

(for shorter table names) in order to fully qualify column

references!

❑ Same rules as with table joins!

❑ You can remove department_id from SELECT clause, it is not

necessary to reference it here for the link

❑ All column values are available in memory, even if not selected!

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❑ Correlated

subquery

using table

aliases to fully

qualify column

references

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❑ Calculate how

many times

an employee

switched jobs:

❑ Note sorting

takes place

on subquery

column using

column alias!

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❑ Multiple, correlated

subqueries

❑ Show the number of

locations and

departments by

country

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10.3 SUBQUERIES AS FILTER EXPRESSIONS

❑ Most common application of subqueries in WHERE/HAVING:

❑ Condition is made up of column/expr on left side

❑ Relational operator

❑ Values or subquery on the right side

❑ Relational operators are

grouped into two categories:

❑ Important to understand

how many rows a subquery

may potentially return!

❑ Example using scalar

subquery in WHERE clause:

❑ Why scalar? Employee_id

is primary key, at most one

row!

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❑ Rules when using

subqueries as

filter expressions:

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❑ Because of GROUP BY

clause subquery

may return multiple

rows → must use

multiple row operator!

❑ Without GROUP BY →

scalar subquery

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❑ You can use multiple

subqueries, either

nested or multiple

subquery conditions

connected through

Boolean operators

❑ Example shows two

nested subqueries:

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❑ Subquery can also

be used in

HAVING clause

❑ Example: Show all

departments where

the minimum salary

in each department

is greater than the

minimum salary in

department 50:

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❑ Multiple row

operators:

❑ When using multiple-row operators the subquery may return

more than one value, that is, many rows of the same column

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❑ Show all employees who

earn exactly the same

salary as one of the

minimum salary for

each department

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❑ IN operator only allows to

compare the equality of

values.

❑ ANY or ALL operators in

conjunction with greater

or less than operators:

❑ ANY requires only one

value from the list to

satisfy the condition

❑ ALL requires all values

from the list to meet the

condition

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❑ Previous example

can be rewritten

using MAX function:

❑ Probably a better

solution → if your query

complexity allows it!

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❑ Modify example:

❑ Display all employees

whose salary is less

than that of all

purchasing clerks

❑ Fewer results due to

the fact that now

salary must be less

than all salary values

of PU_CLERK

❑ Can be rewritten

using MIN function

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❑ Subquery not

returning any

result is set to

Null

❑ Any condition

involving Null

results in False

❑ Or subquery

returning

actual Null

value

❑ Manager_id

is null for one

employee!

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❑ Not IN needs to check

all records → one

record returns Null

→ condition is False

❑ Rewrite subquery

to exclude Null

values

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❑ Using just IN

works with

Null values

since only one

value must

match

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❑ Multiple-column subquery

using compound WHERE

❑ Show employees having

the same combination

of manager_id and

department_id as

employees 141 and

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❑ This is called pairwise

comparison, each row

of the main query

must match the row of

the subquery

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❑ Column comparison

methods:

Pairwise Comparison:

❑ Valid combinations of manager_id

and department_id:

100,20 OR 105,35 OR 121,45

❑ Implementation:

Non-Pairwise Comparison:

❑ Valid combinations:

100,20 OR 100,35 OR 100,45

105,20 OR 105,35 OR 105,45

121,20 OR 121,35 OR 121,45

❑ Implementation:

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❑ Non-Pairwise

comparison

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10.4 SUBQUERIES AS DATASOURCE EXPRESSIONS

❑ Subquery as datasource in FROM clause, also called inline view:

❑ Break down complex queries into different steps

❑ Change SQL processing order by forcing subquery processing

❑ Rewrite previous example

of income categories

and GROUP BY clause:

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10.4 SUBQUERIES AS DATASOURCE EXPRESSIONS

❑ Show last_name, salary, department_id and average salary per

department

❑ Cannot be done in regular

aggregate query since

last_name and salary is not

included in GROUP BY

❑ Use subquery to perform

aggregation per department

❑ Use outer query to add

last_name and salary and

join on subquery

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10.5 CORRELATED SUBQUERIES

❑ Correlated or linked subquery works fundamentally different

from the simple or nested subqueries

❑ Correlated subquery references a column of the main query to

establish a link to the main query

❑ Nested/Simple

Subquery:

❑ Correlates Subquery:

❑ Correlated subquery

can be used as

column, filter,

and as FROM

clause expression

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❑ Show employees

whose salary is

greater than the

average salary in

their respective

department

❑ Similar to previous

example

❑ Performance wise

probably worse since

average salary is

calculated for every

row of the outer

query

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❑ Show employees who have switched jobs at least twice

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❑ Show all managers

❑ Use correlated

subquery in WHERE

clause and EXISTS

operator

❑ EXISTS tests for

mere existence of

a record:

❑ If record exists → TRUE

❑ If no record exists → FALSE

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❑ Instead of EXISTS

use IN operator

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❑ Find all departments

that do not have any

employees

associated with it

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❑ Instead of NOT EXISTS,

use NOT IN

❑ Again, be aware of the

NULL issue!

❑ Need to add condition

in subquery to exclude

NULL values

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Advanced Subqueries

11.

11.1 PIVOT

❑ Use the PIVOT operator to display data horizontally

❑ Oracle PIVOT clause allows you to write a cross-tabulation query

❑ This means that

you can aggregate

your results and

rotate rows into

columns

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11.1 PIVOT

❑ Previous

example

rewritten

using

PIVOT operator

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11.2 SUBQUERY FACTORING

❑ Performance improvement

can be achieved by using the

WITH clause inside a query

block

❑ Useful when a SQL statement

needs to be evaluated more than once within one statement

❑ Rules:

❑ Next Example:

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❑ Rewrite previous query

using WITH clause:

❑ Show last_name, salary,

department_id and average

salary per department

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11.3 TOP-N-ANALYSIS

❑ Retrieves the greatest or least n values based on a particular

sort order

❑ Use ROWNUM pseudo

column to implement

Top-N-Analysis

❑ Use subquery with

desired sort order

❑ Use ROWNUM in main

query to limit to n

rows

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Documents

MANAGING DATA(BASES) USING SQL (NON-PROCEDURAL SQL, …ucb-access.org/Files/SQL/PPPresentation/PPSQLClass7.pdf · 2018-08-17 · 10.2 SUBQUERIES AS COLUMN EXPRESSIONS Must link departments.department_id