Microsoft SQL Server

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Current state

Currently the MSSQL database layer is able to get through an installation and then render the skin and some special pages. It breaks on many operations though such as creating a new user. Login and logout of existing users is working though. Currently there are no indexes in the database, transactions are disabled and unicode is replaced with question marks to be stored.

  • The unicode problem seems to be with the PHP library, using ODBTP may be the solution


Syntax differences between MySQL and MSSQL

MySQL MSSQL
AUTO_INCREMENT IDENTITY(1,1)
binary varchar NULL (MSSQL doesn't allow setting of binary's to string values, and won't implicitly allow NULL's)
bool bit
[UN]SIGNED not valid
SELECT * FROM foo LIMIT x SELECT TOP x * FROM foo
SELECT * FROM foo LIMIT x,y not sure how to implement yet because it must be applied from within DatabaseMssql::limitResult
INSERT IGNORE INTO foo (foo_id,bar) VALUES ('1','xyz') IF NOT EXISTS (SELECT * FROM foo WHERE foo_id = '1') INSERT INTO foo (foo_id,bar) VALUES ('1','xyz')
IF(cond,trueVal,falseVal) CASE WHEN cond THEN trueVal ELSE falseVal END
SHOW TABLES SELECT * FROM INFORMATION_SCHEMA.TABLES
ENUM not natively supported, change to text

MSSQL Variables

@@VERSION Server version information
@@IDENTITY Last inserted row
@@ERROR Last error number

Changes to INSERT wrapper

AUTOINCREMENT vs IDENTITY

MySQL style AUTOINCREMENT columns are inplemented in MSSQL using IDENTITY(x,y) where x is the initial value and y is the amount to add on each insert. The last value resulting from an insert into an IDENTITY column is stored in the @@IDENTITY variable. These kinds of columns are usually used as primary keys and are therefore assigned the NOT NULL property.

In MySQL the standard way of inserting data into rows exhibiting AUTOINCREMENT columns is simply to use a NULL value which will be ignored. In MSSQL however assigning a NULL to an IDENTITY column is not allowed, instead the best way is not to include those items in the list of columns to be updated at all.

To get round this in the MediaWiki MSSQL layer, I've modified the insert wrapper in the DatabaseMssql class to check if the primary key is used in the insert and remove it if so. It checks this by assuming that the primary key will be of the same name as the table but with _id on the end, and that it will the first item in the list of columns to update.

IGNORE

As you can see from the comparison table above, the MySQL INSERT IGNORE option takes quite a different form in MSSQL. This is handled in the insert wrapper method. In the case of multiple row inserts, a separate conditional insert query is performed for each item.

NULL values and NOT NULL columns

MySQL implicitly casts NULL assignments to NOT NULL columns to an empty string or zero value accordingly, but MSSQL raises an error instead. This is a big problem within the MediaWiki environment because the code relies heavily on this implicit NULL casting. I've tried to get round the problem by replacing NULL's with empty strings from update and insert queries, and MSSQL is happy to cast the empty string to a numeric zero if necessary.

See also