Formatting Clear and readable SQL queries - sql

Question

I'm writing some SQL queries with several subqueries and lots of joins everywhere, both inside the subquery and the resulting table from the subquery.
We're not using views so that's out of the question.
After writing it I'm looking at it and scratching my head wondering what it's even doing cause I can't follow it.
What kind of formatting do you use to make an attempt to clean up such a mess? Indents perhaps?

Answer 1

With large queries I tend to rely a lot on named result sets using WITH. This allows to define the result set beforehand and it makes the main query simpler. Named results sets may help to make the query plan more efficient as well e.g. postgres stores the result set in a temporary table.
Example:
WITH
cubed_data AS (
SELECT
dimension1_id,
dimension2_id,
dimension3_id,
measure_id,
SUM(value) value
FROM
source_data
GROUP BY
CUBE(dimension1, dimension2, dimension3),
measure
),
dimension1_label AS(
SELECT
dimension1_id,
dimension1_label
FROM
labels
WHERE
object = 'dimension1'
), ...
SELECT
*
FROM
cubed_data
JOIN dimension1_label USING (dimension1_id)
JOIN dimension2_label USING (dimension2_id)
JOIN dimension3_label USING (dimension3_id)
JOIN measure_label USING (measure_id)
The example is a bit contrived but I hope it shows the increase in clarity compared to inline subqueries. Named result sets have been a great help for me when I've been preparing data for OLAP use. Named results sets are also must if you have/want to create recursive queries.
WITH works at least on current versions of Postgres, Oracle and SQL Server

Answer 2

Boy is this a loaded question. :) There are as many ways to do it right as there are smart people on this site. That said, here is how I keep myself sane when building complex sql statements:
select
c.customer_id
,c.customer_name
,o.order_id
,o.order_date
,o.amount_taxable
,od.order_detail_id
,p.product_name
,pt.product_type_name
from
customer c
inner join
order o
on c.customer_id = o.customer_id
inner join
order_detail od
on o.order_id = od.order_id
inner join
product p
on od.product_id = p.product_id
inner join
product_type pt
on p.product_type_id = pt.product_type_id
where
o.order_date between '1/1/2011' and '1/5/2011'
and
(
pt.product_type_name = 'toys'
or
pt.product_type_name like '%kids%'
)
order by
o.order_date
,pt.product_type_name
,p.product_name
If you're interested, I can post/send layouts for inserts, updates and deletes as well as correlated subqueries and complex join predicates.
Does this answer your question?

Answer 3

Generally, people break lines on reserved words, and indent any sub-queries:
SELECT *
FROM tablename
WHERE value in
(SELECT *
FROM tablename2
WHERE condition)
ORDER BY column

Answer 4

In general, I follow a simple hierarchical set of formatting rules. Basically, keywords such as SELECT, FROM, ORDER BY all go on their own line. Each field goes on its own line (in a recursive fashion)
SELECT
F.FIELD1,
F.FIELD2,
F.FIELD3
FROM
FOO F
WHERE
F.FIELD4 IN
(
SELECT
B.BAR
FROM
BAR B
WHERE
B.TYPE = 4
AND B.OTHER = 7
)

Answer 5

Table aliases and simple consistency will get you a long, long way
What looks decent is breaking lines on main keywords SELECT, FROM, WHERE (etc..).
Joins can be trickier, indenting the ON part of joins brings out the important part of it to the front.
Breaking complicated logical expressions (joins and where conditions both) on the same level also helps.
Indenting logically the same level of statement (subqueries, opening brackets, etc)
Capitalize all keywords and standard functions.
Really complex SQL will not shy away from comments - although typically you find these in SQL scripts not dynamic SQL.
EDIT example:
SELECT a.name, SUM(b.tax)
FROM db_prefix_registered_users a
INNER JOIN db_prefix_transactions b
ON a.id = b.user_id
LEFT JOIN db_countries
ON b.paid_from_country_id = c.id
WHERE a.type IN (1, 2, 7) AND
b.date < (SELECT MAX(date)
FROM audit) AND
c.country = 'CH'
So, at the end to sum it up - consistency matters the most.

Answer 6

I like to use something like:
SELECT col1,
col2,
...
FROM
MyTable as T1
INNER JOIN
MyOtherTable as T2
ON t1.col1 = t2.col1
AND t1.col2 = t2.col2
LEFT JOIN
(
SELECT 1,2,3
FROM Someothertable
WHERE somestuff = someotherstuff
) as T3
ON t1.field = t3.field

Answer 7

The only true and right way to format SQL is:
SELECT t.mycolumn AS column1
,t.othercolumn AS column2
,SUM(t.tweedledum) AS column3
FROM table1 t
,(SELECT u.anothercol
,u.memaw /*this is a comment*/
FROM table2 u
,anothertable x
WHERE u.bla = :b1 /*the bla value*/
AND x.uniquecol = :b2 /*the widget id*/
) v
WHERE t.tweedledee = v.anothercol
AND t.hohum = v.memaw
GROUP BY t.mycolumn
,t.othercolumn
HAVING COUNT(*) > 1
;
;)
Seriously though, I like to use WITH clauses (as already suggested) to tame very complicated SQL queries.

Answer 8

Put it in a view so it's easier to visualize, maybe keep a screenshot as part of the documentation. You don't have to save the view or use it for any other purpose.

Answer 9

Indenting certainly but you can also split the subqueries up with comments, make your alias names something really meaningful and specify which subquery they refer to e.g. innerCustomer, outerCustomer.
Common Table Expressions can really help in some cases to break up a query into meaningful sections.

Answer 10

An age-old question with a thousand opinions and no one right answer, and one of my favorites. Here's my two cents.
With regards to subqueries, lately I've found it easier to follow what's going on with "extreme" indenting and adding comments like so:
SELECT mt.Col1, mt.Col2, subQ.Dollars
from MyTable1 mt
inner join (-- Get the dollar total for each SubCol
select SubCol, sum(Dollars) Dollars
from MyTable2
group by SubCol) subQ
on subQ.SubCol = mt.Col1
order by mt.Col2
As for the other cent, I only use upper case on the first word. With pages of run-on queries, it makes it a bit easier to pick out when a new one starts.
Your mileage will, of course, vary.

Answer 11

Wow, alot of responses here, but one thing I haven't seen in many is COMMENTS! I tend to add a lot of comments throughout, especially with large SQL statements. Formatting is important, but well placed and meaningful comments are extremely important, not just for you but the poor soul who needs to maintain the code ;)