SQL Cheat Sheet_ Query by Example

7/30/2019 SQL Cheat Sheet_ Query by Example

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Copyright 2005 Paragon Corporation ( July 02, 2005)

Structured Query Language (SQL) is a set-based language as opposed to a procedural language.It is the defacto language of relational databases.

The difference between a set-based language vs. a procedural language is that in a set-basedlanguage you define what set of data you want or want to operate on and the atomic operation toapply to each element of the set. You leave it up to the Database process to decide how best tocollect that data and apply your operations. In a procedural language, you basically map out stepby step loop by loop how you collect and update that data.

There are two main reasons why SQL is often better to use than procedural code.

It is often much shorter to write - you can do an update or summary procedurein one line of code that would take you several lines of procedural.For set-based problems - SQL is much faster processor-wise and IO wise toobecause all the underlining looping iteration is delegated to a database server

process that does it in a very low level way and uses IO/processor moreefficiently and knows the current state of the data - e.g. what other processesare asking for the data.

Example SQL vs. Procedural

If you were to update say a sales person of all customers in a particular region -your procedural way would look something like this

do until eof

if rs("state") = "NH" then

rs("salesperson") = "Mike"

end if

rs.next

loop

The SQL way would be:UPDATE customers SET salesperson = "Mike" WHERE state = "NH"

If you had say 2 or 3 tables you need to check, your procedural quickly becomes

difficult to manage as you pile on nested loop after loop.

In this article we will provide some common data questions and processes that SQLis well suited for and SQL solutions to these tasks. Most of these examples arefairly standard ANSI-SQL so should work on most relational databases such as IBMDBII, PostGreSQL, MySQL, Microsoft SQL Server, Oracle, Microsoft Access, SQLite

Why Use SQL Over procedural?



with little change. Some examples involving subselects or complex joins or the morecomplex updates involving 2 or more tables may not work in less advancedrelational databases such as MySQL, MSAccess or SQLite. These examples are mostuseful for people already familiar with SQL. We will not go into any detail abouthow these work and why they work, but leave it up to the reader as an intellectualexercise.

What customers have bought from us?

SELECT DISTINCT customers.customer_id, customers.customer_name

FROM customers INNER JOIN orders ON customers.customer_id = orders.customer_id

Example: What customers have never ordered anything from us?

SELECT customers.* FROM customers LEFT JOIN orders ONcustomers.customer_id = orders.customer_id WHERE orders.customer_id ISNULL

More advanced example using a complex join: What customers have notordered anything from us in the year 2004 - this one may not work in some

lower relational databases (may have to use an IN clause)SELECT customers.* FROM customers LEFT JOIN orders ON(customers.customer_id = orders.customer_id ANDyear(orders.order_date) = 2004) WHERE orders.order_id IS NULL

Please note that year is not an ANSI-SQL function and that many databases do not support it, but

have alternative ways of doing the same thing.

SQL Server, MS Access, MySQL support year().PostGreSQL you do date_part('year', orders.order_date)

SQLite - substr(orders.order_date,1,4) - If you store the date in form YYYY-MM-DDOracle - EXTRACT(YEAR FROM order_date) or to_char(order_date,'YYYY')

Note: You can also do the above with an IN clause, but an IN tends to be slowerSame question with an IN clauseSELECT customers.* FROM customers WHERE customers.customer_id NOTIN(SELECT customer_id FROM orders WHERE year(orders.order_date) =

2004)

How many customers do we have in Massachusetts and California?SELECT customer_state As state, COUNT(customer_id) As total FROMcustomers WHERE customer_state IN('MA', 'CA') GROUP BY customer_state

What states do we have more than 5 customers?

SELECT customer_state, COUNT(customer_id) As total

List all records from one table that are in another table

What items are in one table that are not in another table?

Fun with Statistics - Aggregates



FROM customers

GROUP BY customer_state

HAVING COUNT(customer_id) > 5

How many states do we have customers in?

SELECT COUNT(DISTINCT customer_state) AS total

FROM customers

Note the above does not work in Microsoft Access or SQLite - they do not support COUNT(DISTINCT ..)

Alternative but slower approach for the above - for databases that don'tsupport COUNT(DISTINCT ..), but support derived tables

SELECT count(customer_state) FROM (SELECT DISTINCT customer_state FROM customers);

List in descending order of orders placed customers that have placed more than5 orders

SELECT customer_id, customer_name, COUNT(order_id) as total

FROM customers INNER JOIN orders ON customers.customer_id = orders.customer_id

GROUP BY customer_id, customer_name

HAVING COUNT(order_id) > 5

ORDER BY COUNT(order_id) DESC

Value Insert

INSERT INTO customers(customer_id, customer_name)

VALUES('12345', 'GIS Experts')

Copy data from one table to another table

INSERT INTO customers(customer_id, customer_name)

SELECT cus_key, cus_name

FROM jimmyscustomers WHERE customer_name LIKE 'B%'

Creating a new table with a bulk insert from another table

SELECT * INTO archivecustomers

FROM jimmyscustomers WHERE active = 0

Update from values

UPDATE customers SET customer_salesperson = 'Billy' WHEREcustomer_state = 'TX'

Update based on information from another tableUPDATE customers SET rating = 'Good' FROM orders WHERE orderdate >'2005-01-01' and orders.customer_id = customers.customer_id

How do you insert records in a table?

How do you update records in a table?



Please note the date format varies depending on the database you are using and what date format you

have it set to.

Update based on information from a derived table

UPDATE customers

SET totalorders = ordersummary.total

FROM (SELECT customer_id, count(order_id) As total

FROM orders GROUP BY customer_id) As ordersummary

WHERE customers.customer_id = ordersummary.customer_id

Please note the update examples involving additional tables do not work in MySQL, MSAccess, SQLite.

MS Access Specific syntax for doing multi-table UPDATE joinsUPDATE customers INNER JOIN orders ON customers.customer_id =orders.customer_id SET customers.rating = 'Good'

MySQL 5 Specific syntax for doing multi-table UPDATE joinsUPDATE customers, orders SET customers.rating = 'Good' WHEREorders.customer_id = customers.customer_id

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PostgreSQL 8.3Cheat Sheet

Summary of new and old PostgreSQL functions and SQL constructscomplete xml query and export, and other new 8.3 features, withexamples.

SQLite If you are looking for a free and lite fairly SQL-92 compliantrelational database, look no further. SQLite has ODBC drivers, PHP 5already comes with an embedded SQLite driver, there are .NETdrivers, freely available GUIs , and this will run on most Oses. All thedata is stored in a single .db file so if you have a writeable folderand the drivers, thats all you need. So when you want somethinglite and don't want to go thru a database server install as you wouldhave to with MySQL, MSSSQL, Oracle, PostgreSQL, or don't have

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The Future ofSQL by CraigMullins

Provides a very good definition of what set-based operations are andwhy SQL is superior for these tasks over procedural, as well as abrief history of the language.

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Article that defines all the components of an SQL statement forgrouping data. We wrote it a couple of years ago, but it is still veryapplicable today.

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Covers MySQL datatypes, standard queries, functions

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This is a great summary of the different offerings of Standard SQL,PostGreSQL, DB2, MSSQL, MySQL, and Oracle. It demonstrates by



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SQL Cheat Sheet_ Query by Example