SQL Dialects

BifrostQL abstracts database-specific SQL generation through the ISqlDialect interface. Each dialect handles identifier quoting, pagination syntax, type mapping, and upsert strategies for its target database.

Supported databases

Database	Package	Provider key	Status
SQL Server	`BifrostQL.SqlServer`	`sqlserver`	Production
PostgreSQL	`BifrostQL.Ngsql`	`postgres`	Production
MySQL	`BifrostQL.MySql`	`mysql`	Production
SQLite	`BifrostQL.Sqlite`	`sqlite`	Experimental

Set the provider in configuration:

{
  "BifrostQL": {
    "Provider": "postgres"
  }
}

Dialect differences

Identifier quoting

Database	Style	Example
SQL Server	Brackets	`[orders].[orderId]`
PostgreSQL	Double quotes	`"orders"."orderId"`
MySQL	Backticks	`orders`.`orderId`
SQLite	Double quotes	`"orders"."orderId"`

Pagination

Database	Syntax
SQL Server	`OFFSET @offset ROWS FETCH NEXT @limit ROWS ONLY`
PostgreSQL	`LIMIT @limit OFFSET @offset`
MySQL	`LIMIT @limit OFFSET @offset`
SQLite	`LIMIT @limit OFFSET @offset`

SQL Server requires an ORDER BY clause for OFFSET/FETCH pagination. BifrostQL adds a default order by primary key when no sort is specified.

Upsert strategy

Database	Implementation
SQL Server	`MERGE ... WHEN MATCHED THEN UPDATE WHEN NOT MATCHED THEN INSERT`
PostgreSQL	`INSERT ... ON CONFLICT (pk) DO UPDATE SET ...`
MySQL	`INSERT ... ON DUPLICATE KEY UPDATE ...`
SQLite	`INSERT OR REPLACE INTO ...`

Type mapping specifics

SQL Server:

money, smallmoney map to Decimal
uniqueidentifier maps to String
nvarchar, nchar, ntext map to String

PostgreSQL:

serial, bigserial are treated as auto-increment
uuid maps to String
jsonb, json map to String
text[], integer[] map to [String], [Int]

MySQL:

tinyint(1) maps to Boolean (MySqlConnector returns bool for this type)
enum columns map to String
json maps to String
Column names are case-insensitive

SQLite:

Type affinity rules apply: INTEGER, REAL, TEXT, BLOB
No native BOOLEAN — uses INTEGER with 0/1
No native DATETIME — stored as text in ISO 8601 format

Installing a dialect

Add the dialect package alongside the core server package:

# SQL Server
dotnet add package BifrostQL.SqlServer

# PostgreSQL
dotnet add package BifrostQL.Ngsql

# MySQL
dotnet add package BifrostQL.MySql

# SQLite
dotnet add package BifrostQL.Sqlite

The dialect is selected at startup based on the Provider configuration value. Each dialect registers its own type mapper and SQL generator with the BifrostQL pipeline.

Writing a custom dialect

Implement the ISqlDialect interface to add support for a new database. The dialect must provide:

Identifier quoting
Parameter placeholder style
Pagination SQL generation
Upsert SQL generation
Type mapper (SQL types to GraphQL types)
Schema reader (database metadata to DbModel)