Microsoft SQL Server
Contents
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 |
|
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.
