dev-notes/docs/databases/database-systems/advanced-sql.md

Advanced SQL

SQL is a composed by:

• Data Manipulation Language (DML)
• Data Definition Language (DDL)
• Data Control Language (DCL)

It also includes:

• View definition
• Integrity & Referential Constraints
• Transactions

Note

: SQL is based on bags (duplicates) adn sets (no duplicates)

Aggregations

Functions that return a single value from a bag of tuples:

• AVG(): return the average value
• MIN(): return the minimum value
• MAX(): return the maximum value
• SUM(): return the sum of values
• COUNT(): count the number of values

Note

: aggregate functions can only be used in the SELECT output list. Note: COUNT, SUM, AVG support DISTINCT.

Group By

Project tuples into subsets and calculate aggregates against each subset.

Note

: Not-aggregated values in SELECT output clause must appear in the GROUP BY clause

SELECT * FROM <table>
GROUP BY <table>.<field>;

Having

Filters results based on aggregation computation. Like WHERE clause for a GROUP BY.

SELECT COUNT(1) AS count FROM <table>
GROUP BY <table>.<field>
HAVING count > 0;

String Operations

Note

: strings are case sensitive and defined with single-quotes (').

LIKE is used for string matching: % matches any substring (including empty strings) while _ matches any single character.

SELECT * FROM <table>
WHERE <table>.<field> LIKE '%@c_';

Other common string functions/operators:

• UPPER(): convert string to uppercase
• LOWER(): convert string to lowercase
• || is used to concatenate two strings.

SELECT * FROM <table>
WHERE <table>.<field> = LOWER(<table>.<field> || '-suffix');

DateTime Operations

DateTime Functions:

• NOW(): get current timestamp
• DATE('<timestamp>'): convert string to date
• UNIX_TIMESTAMP(<date>): convert to unix epoch

Output Redirection

Store query results in another table provided that the number and type of columns is the same.

SELECT * INTO <new_table> FROM <table>; -- write result into new table (must not exist)
INSERT INTO <dest_table> (SELECT * FROM <src_table>); -- write result into existing table

Output Control

ORDER BY sorts the results based on a specific attribute(s).

SELECT * FROM <table> ORDER BY <table>.<field> ASC; -- default direction
SELECT * FROM <table> ORDER BY <table>.<field> DESC;

SELECT * FROM <table> ORDER BY <table>.<field_1> ASC, <table>.<field_2> DESC;

LIMIT limits the number of tuples returned in output. Can set an offset to return a range.

SELECT * FROM <table> LIMIT 10;
SELECT * FROM <table> LIMIT 10 OFFSET 5;

Nested Queries

Inner queries can appear (almost) anywhere in the query. They are often difficult to optimize.

Note

: inner queries can reference attributes and tables of the outer query but not vice-versa.

SELECT * FROM <table> WHERE <table>.<field> IN (SELECT <table.field> ...);
SELECT (SELECT <table>.<field> FROM <table> WHERE ...) FROM <table> WHERE <expr>;

Nested query operators:

• ALL(): must satisfy expression for all rows in sub-query.
• ANY(): must satisfy expression for at least one row in sub-query.
• IN: equivalent to =ANY().
• EXISTS: at least one row is returned.

Window Functions

Perform a calculation across a set of tuples related to a single row. Like an aggregation but tuples are not grouped into a single output tuple.

The OVER keyword specifies how to group together tuples when computing the window function.

SELECT ..., FUNC_NAME(<expr>) 
OVER (
    PARTITION BY <expr>, ...
    ORDER BY <expr>, ...
) 
FROM <table>;

Special window functions:

• ROW_NUMBER(): number of the current row
• RANK(): order position of the current row

Common Table Expressions (CTE)

Provides a way to write auxiliary statements for a larger query. Alternative to nested queries and views.

WITH <cte_name> (<column>, ...) AS (SELECT ...)  -- temporary table from query result
SELECT <column>, ... FROM <cte_name>

Note: CTEs can be recursive with the RECURSIVE keyword.