Friday, May 18, 2007

Quotable Quotes

Spare a kind thought for the hard-working yet underappreciated quotation mark. Single quotes or double quotes, you ask? Exactly!

If you're familiar with other languages such as Perl or Javascript, you probably intermix single and double quotes as needed. Double quotes around strings containing single quotes. Single quotes around Perl strings containing $ or @ characters, so they aren't interpolated as embedded variables.

With PL/SQL, you'll almost always use single quotes. The main exception is around identifier names, which you can enclose in double quotes if they contain special characters or if you need to force them into mixed case or lowercase:

execute immediate 'create table "My_Table" ...';

Without those double quotes, the new table would be known internally (in the USER_TABLES view, etc.) as MY_TABLE. The double quotes force the object name to be exactly as entered, meaning you would get an error from "select * from MY_TABLE" or "select * from my_table". (In this last case, the name my_table is translated silently to MY_TABLE.)

So, that's it then. Use single quotes for string literals, nothing more to learn.

var1 := 'Hello world!';

But what happens when we start dealing with real-world cases?

var2 := 'Oracle Database Administrator''s Guide';

Wait a second! What's with the 2 consecutive single quotes? That's the notation to escape a single quote inside a string literal. What a bother. If you want to generate a text file with a bunch of insert statements, you have to run a search/replace on each string to create 2 of each single quote. And you have to do it before wrapping single quotes around the data. When done in PL/SQL, this leads to one of my favourite PL/SQL statements of all time:

var3 := replace(var2,'''','''''');

We are only replacing 1 single quote with 2 single quotes, but between the surrounding quotes for the literals and the doubled quotes to escape them inside the literals, it looks like an entry in the Obfuscated C Contest.

Luckily, in Oracle 10g there is a way to avoid all the escaping. It still requires you to predict what characters your string might contain, but within that constraint it is much more readable:

var4 := q'{Oracle Database Administrator's Guide}';
var5 := q'[I'm the king of the world!]';
var6 := q'#Mommy's all right, Daddy's all right...#';

With the q literal notation, the first and last characters inside the quotes are not part of the string value, and single quotes are OK since you are making sure that the closing sequence (in the examples above, }', ]', and #') do not appear anywhere in the string. If the opening character after the q' is a "bracket"-style character, the matching end character is the corresponding close bracket. If the opening character is some other punctuation mark or what have you, the same character is used immediately before a single quote to end the literal.

Where this really comes into play is in writing out code of one kind or another from PL/SQL. In a web application, you might be writing out HTML full of double quotes, Javascript full of single quotes, or a combination:

obj.innerHTML = '<div style="float: right;">';

It's very comforting to know you can wrap the whole thing in a PL/SQL string literal without doing any replacements inside the string:

var7 := q'|obj.innerHTML = '<div style="float: right;">';|';

Again, you'll have to know based on your own coding conventions or a priori that the closing delimiter character doesn't appear immediately before a single quote.

I like to combine this technique with a SQL*Plus trick I mentioned in an earlier post:

var8 :=

Now that Javascript is available to PL/SQL and can be plunked into a web page, concatenated with other source, searched/replaced, etc. It doesn't have to contain any escaped '' sequences and so can be reused with other languages, edited by a non-PL/SQL programmer, run through a Javascript validator, or anything else that strikes your fancy.