SQL Patterns in Practice - You're (Probably) Doing it Wrong - Methods for Improving SQL

SQL Patterns
in Practice
(You’re Probably
Doing it Wrong)
Sean Scott, Oracle DBA
sean.scott@bodybuilding.com

The Four Rules of
Writing SQL
To improve readability and self-document code, follow these
four simple rules.

SELECT
order.order_id, order_line, product_name, unit_price, supplier_
name, SUM(total_units), sum(unit_price*total_units)
FROM order, product, order_items
WHERE ((order_items.product_id = product.product_id) and
((order.order_id = order_items.order_id) and (customer_id =
42)))
GROUP BY
order_id, product_name, unit_price, supplier_name, total_units
ORDER BY order_id, product_name;

Rule 1
Everything on its own line
This improves readability and makes troubleshooting easier.

SELECT order.order_id,
order_line,
product_name,
unit_price,
supplier_name,
SUM(total_units), sum(unit_price*total_units)
FROM order,
product,
order_items
((order.order_id = order_items.order_id) and
(customer_id = 42)))
GROUP BY…

SELECT order.order_id,
--order_line,
product_name,
--unit_price,
supplier_name--,
--SUM(total_units),
--sum(unit_price*total_units)
FROM order,
product--,
--order_items
(--(order.order_id = order_items.order_id) and
and (customer_id = 42)))

Rule 2
Put commas, ANDs at the
beginning of lines, not the end
This minimizes commenting necessary to remove something
from your query.

SELECT order.order_id
, order_line
, product_name
, unit_price
, supplier_name
, SUM(total_units)
, sum(unit_price*total_units)
FROM order
, product
, order_items
WHERE ((order_items.product_id = product.product_id)
and ((order.order_id = order_items.order_id)
and (customer_id = 42)))

SELECT order.order_id
--, order_line
, product_name
--, unit_price
, supplier_name
--, SUM(total_units)
--, sum(unit_price*total_units)
FROM order
, product
--, order_items
WHERE ((order_items.product_id = product.product_id)
--and ((order.order_id = order_items.order_id)
( and (customer_id = 42)))

Rule 3
When joining tables, use short,
meaningful aliases for tables
and always prefix columns
It eliminates confusion about what table a column belongs
to, and improves readability.

SELECT o.order_id
, ol.order_line
, p.product_name
, ol.unit_price
, p.supplier_name
, SUM(ol.total_units)
, sum(p.unit_price*ol.total_units)
FROM order o
, product p
, order_items oi
WHERE ((oi.product_id = p.product_id)
and ((o.order_id = oi.order_id)
and (o.customer_id = 42)))

Rule 4
Avoid parentheses in WHERE
clauses unless required to nest
“OR” expressions
Simplify SQL by eliminating that which is unnecessary

SELECT o.order_id
, ol.order_line
, p.product_name
, ol.unit_price
, p.supplier_name
, sum(p.unit_price*ol.total_units)
FROM order o
, product p
, order_items oi
WHERE oi.product_id = p.product_id
and o.order_id = oi.order_id
and o.customer_id = 42

Four Rules
• Everything on its own line
• Put commas, “AND” at the beginning of lines, not
the end
• When joining tables, use short, meaningful aliases
for tables and always prefix columns
• Avoid parentheses in WHERE clauses unless required
to nest “OR” expressions.

The Six Habits of
Legible SQL
Seven habits to adopt when writing SQL that
will improve legibility

Pick a case and stick to it
Upper or lower, it doesn’t matter, provided you’re consistent.
Case provides visual cues about the purpose or meaning of
the various parts of your statement.

SELECT o.order_id
, ol.order_line
, p.product_name
, ol.unit_price
, p.supplier_name
, SUM(p.unit_price*ol.total_units)
FROM order o
, product p
, order_items oi
and o.order_id = oi.order_id
and o.customer_id = 42

Use white space to align
statements for meaning
White space makes the parts and purpose of a statement
more visually apparent and easier to read.

SELECT o.order_id
, ol.order_line
, p.product_name
, ol.unit_price
, p.supplier_name
FROM order o
, product p
, order_items oi
AND o.order_id = oi.order_id
AND o.customer_id = 42
GROUP BY o.order_id
, ol.order_line
...

Use white space to align
operands and aliases
When all operands fall into alignment, it’s much easier to see the
left and right sides in the WHERE statement. Likewise, when table
aliases are aligned it’s much easier to reference them.

Group columns by table
If you need to troubleshoot by commenting out a table, it’s
more efficient when everything is together.

SELECT o.order_id
, p.product_name
, p.supplier_name
, ol.order_line
, ol.unit_price
FROM order o
, product p
, order_items oi
AND o.order_id = oi.order_id
AND o.customer_id = 42
GROUP BY o.order_id
, ol.order_line
...

In a WHERE clause,
equalities first, then IN lists,
and subqueries last.
This orders statements from the least to most likely to cause a
problem and need to be edited.

SELECT o.order_id
, p.product_name
, p.supplier_name
, ol.order_line
, ol.unit_price
FROM order o
INNER JOIN order_items oi
ON o.order_id = oi.order_id
INNER JOIN product p
ON oi.product_id = p.product_id
WHERE o.customer_id = 42
GROUP BY o.order_id
, ol.order_line
...

Place aggregate functions
last in the SELECT
It makes the GROUP BY easier to write—just copy/paste the
SELECT clause up to the aggregates as the GROUP BY.

Six Habits of Legible SQL
• Pick a case and stick to it
• Use white space to align statements for meaning
• Use white space to align operands and aliases
• Group columns by table
• In a WHERE clause, equalities first, then IN lists, and
subqueries last
• Place aggregate functions last in the SELECT

SQL Patterns in Practice - You're (Probably) Doing it Wrong - Methods for Improving SQL

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