ANSI_NULLS and QUOTED_IDENTIFIER killed things. What are they for?

Question

NOTE: I checked Understanding QUOTED_IDENTIFIER and it does not answer my question.

I got my DBAs to run an index I made on my Prod servers (they looked it over and

Answer 1

I find the documentation, blog posts, Stackoverflow answers unhelpful in explaining what turning on QUOTED_IDENTIFIER means.

Olden times

Originally, SQL Server allowed you to use quotation marks ("...") and apostrophes ('...') around strings interchangeably (like Javascript does):

• SELECT "Hello, world!" --quotation mark
• SELECT 'Hello, world!' --apostrophe

And if you wanted a name table, view, procedure, column etc with something that would otherwise violate all the rules of naming objects, you could wrap it in square brackets ([, ]):

CREATE TABLE [The world's most awful table name] (
   [Hello, world!] int
)

SELECT [Hello, world!] FROM [The world's most awful table name]

And that all worked, and made sense.

Then came ANSI

Then ANSI came along and had other ideas:

• if you have a funky name, wrap it in quotation marks ("...")
• use apostrophe ('...') for strings
• and we don't even care about your square brackets

Which means that if you wanted to "quote" a funky column or table name you must use quotation marks:

SELECT "Hello, world!" FROM "The world's most awful table name"

If you knew SQL Server, you knew that quotation marks were already being used to represent strings. If you blindly tried to execute that ANSI-SQL, it is the same as trying to execute:

SELECT 'Hello, world!' FROM 'The world''s most awful table name'

as though it were T-SQL: it's nonsense, and SQL Server tells you so:

Msg 102, Level 15, State 1, Line 8
Incorrect syntax near 'The world's most awful table name'.

You must opt-in to the new ANSI behavior

So Microsoft added a feature to let you opt-in to the ANSI flavor of SQL.

Original (aka SET QUOTED_IDENTIFIER OFF)

SELECT "Hello, world!" --valid
SELECT 'Hello, world!' --valid

SET QUOTED_IDENTIFIER ON

SELECT "Hello, world!" --INVALID
SELECT 'Hello, world!' --valid

These days everyone has SET QUOTED_IDENTIFIERS ON, which technically means you should be using quotes rather than square brackets around identifiers:

T-SQL (bad?) (e.g. SQL generated by Entity Framework)

UPDATE [dbo].[Customers]
SET [FirstName] = N'Ian'
WHERE [CustomerID] = 7

ANSI-SQL (good?)

UPDATE "dbo"."Customers"
SET "FirstName" = N'Ian'
WHERE "CustomerID" = 7