As a DBA, one of the things that happens to me several times a day is finding

a chunk of SQL in my inbox or, worse still, on a piece of paper dropped on my

desk. Yes, it's SQL that performs poorly or doesn't do what the programmer

expected and now I'm asked to look at it. And, it's often the case that this

chunk of SQL is just plain ugly; hard to read and understand. There are two Best

Practices that frequently get applied to such messes before I really start

analyzing the problem…

BEST PRACTICE 1 - Use Mnemonic Table Aliases.

I found this chunk of SQL in a Sybase group today:

select distinct
a.clone_id,b.collection_name,a.source_clone_id,a.image_clone_id,c.library_name,c.vector_name,
c.host_name,d.plate,d.plate_row,d.plate_column,a.catalog_number,a.acclist,a.vendor_id,b.value,c.species,e.cluster
          from clone a,collection b,library c,location d, sequence e
         where a.collection_id = b.collection_id
           and a.library_id = c.source_lib_id
           and a.clone_id = d.clone_id
           and a.clone_id = e.clone_id
           and b.short_collection_type='cDNA'
           and b.is_public = 1
           and a.active = 1
           and a.no_sale = 0
           and e.cluster in (select cluster from master_xref_new where
type='CLONE' and id='LD10094')

I'm sure the news client has damaged the formatting of this a little bit but

it's still obvious that the programmer didn't put any effort into making this

SQL readable and easy to understand. And there it was in the newsgroups and he

wanted us to read and understand it. Wonderful.

For me, the worst part of this query are the table aliases. A, B, C, D, E. I

find that I must continually refer back to the "from" clause to try and remember

what the heck A or E or whatever represents. Figuring out whether or not the

programmer has gotten the relationships right is a real pain in the neck with

this query. He's saved typing, sure, but at a tremendous cost in clarity. And

I've had much worse end up on my desk: tables from A to P on at least one

occasion and about three pages long, with some columns in the SELECT list that

weren't qualified by table aliases at all.

Let's rewrite this guy's query for him using this first Best Practice (I'm

not going to do anything about his spacing):

select distinct
clo.clone_id,clc.collection_name,clo.source_clone_id,clo.image_clone_id,lib.library_name,lib.vector_name,
lib.host_name,loc.plate,loc.plate_row,loc.plate_column,clo.catalog_number,clo.acclist,clo.vendor_id,clc.value,lib.species,seq.cluster
         from clone clo,collection clc,library lib,location loc, sequence seq
         where clo.collection_id = clc.collection_id
           and clo.library_id = lib.source_lib_id
           and clo.clone_id = loc.clone_id
           and clo.clone_id = seq.clone_id
           and clc.short_collection_type='cDNA'
           and clc.is_public = 1
           and clo.active = 1
           and clo.no_sale = 0
           and seq.cluster in (select cluster from master_xref_new where 
type='CLONE' and id='LD10094')

Without bothering to fix the spacing, isn't this already easier to

understand? Which query lends itself to easier maintenance? Trust me, it's the

latter, every time.

In some situations, being able to easily identify the source table for a

column in the select list can be a big help, too. You may have two different

tables which have fields with identical names but which mean different things.

Catching those will be easier with mnemonics.

We can make another big improvement in this query with another best

practice...

BEST PRACTICE 2 - Use ANSI JOIN Syntax

Do this to clearly demonstrate the separation between "How do we relate these

tables to each other?" and "What rows do we care about in this particular

query?"

In this case, I can only guess what the programmer is up to but, if I were a

DBA at his site and knew the relationships between the tables, I could use this

"relating" vs. "qualifying" dichotomy to help troubleshoot his queries. Let's

rewrite this query again (but I'm still not going to do much about his spacing):

select distinct
clo.clone_id,clc.collection_name,clo.source_clone_id,clo.image_clone_id,lib.library_name,lib.vector_name,
lib.host_name,loc.plate,loc.plate_row,loc.plate_column,clo.catalog_number,clo.acclist,clo.vendor_id,clc.value,lib.species,seq.cluster
          from clone clo
inner join collection clc
on clo.collection_id = clc.collection_id
inner join library lib
on clo.library_id = lib.source_lib_id
inner join location loc
on clo.clone_id = loc.clone_id
inner join sequence seq
on clo.clone_id = seq.clone_id
         where clc.short_collection_type='cDNA'
           and clc.is_public = 1
           and clo.active = 1
           and clo.no_sale = 0
           and seq.cluster in (select cluster from master_xref_new where 
type='CLONE' and id='LD10094')

I still can't say for sure that this query is right. However, the DBA that

does know this database is going to find it much easier to spot a missing

element of the relationship between, say, collection and clone. It's certainly

much easier to spot a situation where the programmer failed to include any

relationship to one of the tables (it would be obvious to us at this point), so

you get fewer accidental Cartesian Products.

In my experience, simply rewriting ugly queries according to these best

practices has often pointed up the nature of the problem and made the solution a

snap. This certainly happens often enough that taking the time to do the rewrite

is worth the trouble.

Another advantage of following this rule is that it allows you to readily

steal an important chunk of your SQL statements from any nearby statement that

already relates these tables. Just grab the FROM clause out of another

statement, put in the WHERE that's customized for this situation and you're

ready, with some confidence, to run the query. Being a lazy sort, this feature

is a real plus for me.

So, encourage mnemonic table aliases and use of ANSI JOIN syntax. As Red

Green says: "I'm pullin' for ya. We're all in this together." He's right; your

programmers might end up at my site or vice-versa someday.