Nested Queries Lecture

Nested Queries
Felipe dos Santos Costa
fsc7@yahoo.com
May 2016

Topics
●
Nested queries
●
CREATE
●
UPDATE
●
DELETE
●
The Subquery as Scalar Operand
●
Comparisons Using Subqueries
●
Subqueries with ANY, IN, or SOME
●
Subqueries with ALL
●
Row Subqueries
●
Subqueries with EXISTS or NOT EXISTS
●
Correlated Subqueries
●
Subqueries in the FROM Clause
●
Subquery Errors
●
Optimizing Subqueries
●
Rewriting Subqueries as Joins

What are Nested Queries?
● A Subquery or Inner query or Nested query is a query within
another SQL query and embedded within the WHERE clause.
● Subqueries (also known as inner queries or nested queries) are
a tool for performing operations in multiple steps.
– Subqueries can be used in several places within a query, but
it’s easiest to start with the FROM statement.
●
Subqueries can return individual values or a list of records

Example
● SELECT * FROM t1 WHERE column1 = (SELECT column1 FROM t2);
– Outer query Subquery

Advantages
● The main advantages of subqueries are:
– They allow queries that are structured so that it is
possible to isolate each part of a statement.
– They provide alternative ways to perform operations
that would otherwise require complex joins and unions.
– More readable than complex joins or unions.
● Indeed, it was the innovation of subqueries that gave
people the original idea of calling the early SQL
“Structured Query Language.”

Return
● Scalar (a single value)
● Single row
● Single column
● Table (one or more rows of one or more columns).

Other info
● Subqueries must be enclosed with parenthesis
● A subquery can contain many of the clauses that an
ordinary SELECT can: DISTINCT, GROUP BY, ORDER BY,
LIMIT, joins, index hints, UNION constructs, comments,
functions, and so on.
● A subquery's outer statement can be any one of: SELECT,
INSERT, UPDATE, DELETE, SET, or DO.
● In MySQL, you cannot modify a table and select from the
same table in a subquery

Database used for example
● Historical series of fertility rate

CREATE
● CREATE TABLE countries AS (SELECT DISTINCT country
from fertility)

UPDATE
● How can I get the updated fertility rate for each country?
● SELECT country, year, fertility FROM fertility WHERE year = 2015
GROUP BY country
● How can I update my country table using subquery?
● UPDATE countries c SET c.fertility = (
SELECT fertility
FROM fertility f
WHERE YEAR =2015
AND f.country = c.country
GROUP BY country),
c.year =2015

Another example
● Two sources of data

UPDATE
● How to get one column (continent) from the second
source?
● UPDATE countries c SET c.continent =
(SELECT continent_code FROM countries2 c2
WHERE c2.name = c.country )

DELETE
● How to delete countries which has no data
● DELETE FROM countries WHERE country IN (SELECT
country FROM fertility GROUP BY country HAVING
SUM(fertility) IS NULL) *
● * It might give error on MySQL workbench because of
safe update mode. (Error Code: 1175) – You must disable
safe mode.

The Subquery as Scalar Operand
● A scalar subquery is a simple operand, and you can use it almost
anywhere a single column value or literal is legal, and you can expect
it to have those characteristics that all operands have: a data type, a
length, an indication that it can be NULL, and so on. For example:
● CREATE TABLE t1 (s1 INT, s2 CHAR(5) NOT NULL);
● INSERT INTO t1 VALUES(100, 'abcde');
● SELECT (SELECT s2 FROM t1);
● The subquery in this SELECT returns a single value ('abcde') that has
a data type of CHAR, a length of 5, a character set and collation
equal to the defaults in effect at CREATE TABLE time, and an
indication that the value in the column can be NULL.

Example
● Fertility average of Europe and South America
– SELECT (SELECT AVG(fertility) FROM countries
WHERE continent = 'EU'),
(SELECT AVG(fertility) FROM countries WHERE
continent = 'SA')

Comparisons Using Subqueries
● = > < >= <= <> != <=>
● What are the countries in South America which have
fertility smaller than Max european fertility?
● SELECT * FROM countries
WHERE fertility < (
SELECT MAX(fertility)
FROM countries
WHERE continent='EU')
AND continent = 'SA';

Subqueries with ANY, IN, or SOME
● Which countries in Africa have fertility rate < than any
european country?
● SELECT * FROM countries
WHERE fertility < ANY
(SELECT fertility FROM countries WHERE
continent='EU')
AND continent = 'AF';
● When used with a subquery, IN is an alias for = ANY

Subqueries with ALL
● SELECT s1 FROM t1 WHERE s1 > ALL
(SELECT s1 FROM t2);
● Which countries in Africa have the fertility rate bigger
than all countries of Asia and South America
● SELECT s1 FROM t1 WHERE s1 <> ALL
(SELECT s1 FROM t2);
● NOT IN is an alias for <> ALL

Row Subqueries
● A row subquery is a subquery variant that returns a single
row and can thus return more than one column value
● SELECT * FROM t1 WHERE (col1, col2) = (SELECT col3,
col4 FROM t2 WHERE id = 10);
● Which countries are on the average of 2015?
● SELECT country, year, fertility FROM fertility
WHERE (TRUNCATE(fertility,1), year) =
(SELECT TRUNCATE(AVG(fertility),1), year FROM
countries);

Row Subqueries
● SELECT * FROM t1 WHERE ROW(col1, col2) = (SELECT
col3, col4 FROM t2 WHERE id = 10);
● The row constructor and the row returned by the subquery
must contain the same number of values.
● The following query answers the request, “find all rows in
table t1 that also exist in table t2”:
● SELECT column1, column2, column3
FROM t1
WHERE (column1, column2, column3) IN
(SELECT column1, column2, column3 FROM t2);

Row Subqueries
● Which countries have the same fertility rate than Estonia
(rounding)?
● SELECT * FROM fertility
WHERE ROW(TRUNCATE(fertility,1), year) =
(SELECT TRUNCATE(fertility,1), year
FROM countries
WHERE country='Estonia');

Subqueries with EXISTS or NOT EXISTS
● SELECT column1 FROM t1 WHERE EXISTS (SELECT * FROM t2);
● What countries exists in my two datasources?
– SELECT DISTINCT country FROM countries c
WHERE EXISTS (SELECT name FROM countries2 c2 WHERE
c.country = c2.name);
● Now the opposite
– SELECT DISTINCT country FROM countries c
WHERE NOT EXISTS (SELECT name FROM countries2 c2
WHERE c.country = c2.name);

● A correlated subquery is a subquery that contains a reference to
a table that also appears in the outer query.
● SELECT * FROM t1
WHERE column1 = ANY (SELECT column1 FROM t2
WHERE t2.column2 = t1.column2);
● In which years Estonia had fertility rate bigger than it's historical
average.
● SELECT * FROM fertility f
WHERE fertility >
(SELECT AVG(fertility) FROM fertility f2 WHERE f.country =
f2.country AND f2.country = 'Estonia' GROUP BY f2.country);

● In which years Estonia had fertility smaller than average
for 2000 to 2015 (1.53)?
● SELECT * FROM fertility f
WHERE fertility <
(SELECT AVG(fertility)
FROM fertility f2
WHERE f.country = f2.country AND f2.country = 'Estonia'
AND f2.year BETWEEN 2000 AND 2015
GROUP BY f2.country);

Subqueries in the FROM Clause
● SELECT ... FROM (subquery) [AS] name …
● Average of fertility for each continent using historical average
for each country
●
SELECT continent, AVG(avg_fertility)
FROM
(SELECT AVG(fertility) as avg_fertility, country
FROM fertility f
WHERE year BETWEEN 2000 AND 2015
GROUP BY country) AS avgfert
JOIN countries c ON (c.country = avgfert.country)
GROUP BY continent

If we have time
● https://www.google.com/fusiontables/DataSource?doc
id=1tVN1toVTUb1Ju3gaLxIHTtlcST_bdaR7UgU2OfJO#rows:
id=1
● https://www.google.com/fusiontables/DataSource?do
cid=1kg8Pn9JEheqA8whqsmZBgM3quEiPTyFrasfUv5hQ

References
● MySQL 5.7 Reference Manual - http://dev.mysql.com/doc/refman/5.7/en/
● Data Country and Continents
http://www.geekality.net/2011/08/21/country-names-continent-names-
and-iso-3166-codes-for-mysql/
● Historical Fertility Rate - http://www.gapminder.org/data/
https://ourworldindata.org/grapher/total-fertility-rate?tab=map
● Nested Queries
http://www.w3resource.com/sql/subqueries/nested-subqueries.php
● Subqueries -
https://sqlschool.modeanalytics.com/advanced/subqueries/
● Using Nested Queries -
http://sqlzoo.net/wiki/Using_nested_SELECT

Nested Queries Lecture

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Nested Queries Lecture