Writing Database Queries Skill for Claude

Dual-ORM Architecture

Bitwarden maintains two data access implementations, split by database provider:

• MSSQL: Dapper with stored procedures
• PostgreSQL, MySQL, SQLite: Entity Framework Core

These implementations are mutually exclusive at runtime — SQL Server uses only Dapper, while the other providers use only EF Core. Both implementations conform to the same repository interfaces.

• When adding new repository functionality, implement it in both Dapper and EF Core (unless the feature is explicitly EF-only).
• When modifying an existing stored procedure in a backwards-compatible way (for example, adding a new parameter with a default), EF Core changes are not required.
• Some commercial features (for example, Secrets Manager) are EF Core only.

Evolutionary Database Design (EDD)

Bitwarden Cloud uses a no-rollback approach to database deployments. The key implication: server deployments can be rolled back, but database migrations cannot, so migrations must be designed to avoid being a source of downtime.

All MSSQL migrations live in util/Migrator/DbScripts/ and execute in chronological order based on the migration filename (YYYY-MM-DD_##_Description.sql).

> Note: You may see util/Migrator/DbScripts_transition/ and util/Migrator/DbScripts_finalization/ folders. These are not currently used; ignore them for now.

Simple additive changes (new nullable column, new table, new stored procedure) typically require only a single migration script in util/Migrator/DbScripts/.

Stored procedure compatibility

Stored procedure changes fall into two categories:

• Non-breaking (DEFAULT parameters): Adding a parameter with a default value (e.g., @NewParam BIT = NULL) is backwards-compatible. Existing callers keep working; no _V2 is needed.
• Breaking (_V2 versioning): Required when result-set structure changes, calling patterns change (e.g., single result → multiple result sets), required parameters are added without defaults, or query semantics differ. Implement this by creating ProcedureName_V2 while retaining the original procedure for backwards compatibility.

Table-level breaking changes (removing columns, changing types) typically cascade into stored procedure changes and often require the _V2 pattern.

Always defer to the developer on migration strategy. The approach is complex and context-dependent. When a database change is needed, write the migration script and ask the developer whether _V2 versioning or additional steps are required.

Key locations

• src/Sql/dbo — Master schema source of truth
• util/Migrator/DbScripts — All migrations (single folder, chronological)

ORM-Specific Implementation

When implementing Dapper repository methods, stored procedures, or MSSQL migration scripts, activate the implementing-dapper-queries skill.

When implementing EF Core repositories, generating EF migrations, or working with PostgreSQL/MySQL/SQLite, activate the implementing-ef-core skill.

Critical Rules

These are the most frequently violated conventions. Claude cannot fetch the linked docs at runtime, so these are inlined here:

• Migration file naming: YYYY-MM-DD_##_Description.sql (e.g., 2025-06-15_00_AddVaultColumn.sql)
• All schema objects use dbo schema — never create objects in other schemas
• Constraint naming: PK_TableName (primary key), FK_Child_Parent (foreign key), IX_Table_Column (index), DF_Table_Column (default)
• Idempotent scripts: Use IF NOT EXISTS / IF COL_LENGTH(...) guards before schema changes in migration scripts
• New repository functionality requires both Dapper and EF Core implementations — unless the feature is explicitly EF-only or the change is a backwards-compatible stored procedure modification
• Integration tests use [DatabaseData] attribute — this runs the test against all configured database providers

Further Reading

• SQL code style
• Database migrations (MSSQL)
• Database migrations (EF)
• Evolutionary Database Design