SQL Skills

A Claude Code and Codex plugin containing 32 agent skills for writing correct, portable, modern standard SQL (ISO/IEC 9075).

The skills are vendor-neutral. They teach the standard spelling of each feature, name the correctness traps that LLMs reliably fall into, and map portability per feature across PostgreSQL, MySQL/MariaDB, SQLite, SQL Server, Oracle, and DuckDB — so the SQL an agent writes is right the first time and runs where you need it.

What this is for

Most SQL goes wrong in the same handful of places: a filter on the null-able side of an outer join silently demoting it to an inner join; NOT IN collapsing to zero rows because one value was NULL; FLOAT chosen for money; string-interpolated queries inviting injection. These skills auto-invoke when Claude is about to write or edit the relevant SQL and steer it onto the standard, safe path.

Each skill is self-contained: a SKILL.md with the rule and worked examples, plus a references/common-mistakes.md catalog of the specific antipatterns it guards against.

Installation

Add the marketplace, then install the sql plugin:

/plugin marketplace add ctoth/sql-skills-plugin
/plugin install sql@sql-skills-marketplace

The skills activate automatically based on what you're writing — no commands to remember. You can also browse them with /plugin.

For Codex:

codex plugin marketplace add ctoth/sql-skills-plugin
codex plugin add sql@sql-skills-marketplace

Marketplace registration and plugin installation are separate operations.

Skills

Foundations — the correctness floor

Skill	Covers
sql-relational-and-null-discipline	Results are unordered sets; NULL means unknown and propagates UNKNOWN; WHERE/HAVING/ON keep only TRUE, CHECK rejects only FALSE.
sql-select-and-query-processing	Logical clause-evaluation order (FROM → WHERE → GROUP BY → HAVING → SELECT → DISTINCT → UNION → ORDER BY → FETCH) and what each clause can reference.
sql-style-and-naming	Readable, reviewable SQL — plus the 'literal' vs "identifier" quoting trap.

Querying

Skill	Covers
sql-joins	Join composition and the outer-join-filter-in-WHERE bug that silently drops rows.
sql-subqueries-and-exists	Correlated subqueries and the NOT IN + NULL trap; NOT EXISTS as the safe anti-join.
sql-aggregation-and-grouping	The functional-dependency GROUP BY rule, WHERE vs HAVING, FILTER (WHERE …), GROUPING SETS/ROLLUP/CUBE.
sql-window-functions	Window functions and the frame-clause traps (ROWS vs RANGE, default frames).
sql-set-operations	UNION vs UNION ALL (and the dedup cost trap), INTERSECT, EXCEPT.
sql-cte-and-recursion	WITH for decomposition and WITH RECURSIVE with termination/cycle guards.
sql-lateral-and-correlated-derived	LATERAL — a FROM-clause derived table that can reference preceding items.
sql-expressions-case-and-functions	Portable scalar expressions: CASE, COALESCE, NULLIF, `
sql-pattern-matching-and-collation	LIKE/ESCAPE and how collation silently changes case sensitivity across engines.

Data types & expressions

Skill	Covers
sql-data-types-and-numerics	Type selection by meaning — exact NUMERIC(p,s) for money, never binary FLOAT.
sql-datetime-and-intervals	Temporal types, TIMESTAMP WITH TIME ZONE, typed literals, intervals.
sql-json	Standard SQL/JSON (JSON_VALUE/JSON_QUERY/JSON_EXISTS/JSON_TABLE) over vendor operators.

Writing data

Skill	Covers
sql-data-modification	INSERT/UPDATE/DELETE as set-based operations, not row-by-row loops.
sql-merge-and-upsert	Atomic upsert instead of a race-prone SELECT-then-INSERT-or-UPDATE.
sql-generated-and-identity-columns	GENERATED … AS IDENTITY and computed columns over vendor SERIAL/AUTO_INCREMENT.

Schema & integrity

Skill	Covers
sql-schema-design-and-normalization	The 1NF→BCNF ladder, natural vs surrogate keys, junction tables, deliberate denormalization.
sql-constraints-and-integrity	Push integrity into the schema as declarative constraints, not app-side re-checks.
sql-views-and-introspection	Views as stored queries, updatability rules, CHECK OPTION, and INFORMATION_SCHEMA.

Performance & scale

Skill	Covers
sql-indexing-and-sargability	Index design and the sargability rule — keep the indexed column bare in predicates.
sql-explain-and-set-based-thinking	Think in sets, not rows; measure the plan instead of guessing.
sql-pagination-and-keyset	Scalable keyset pagination and the row-value machinery behind it.

Security & transactions

Skill	Covers
sql-injection-and-parameterization	The non-negotiable rule: bind every value; never interpolate SQL.
sql-privileges-and-access-control	GRANT/REVOKE, roles, and default-deny access.
sql-transactions-and-isolation	Transaction boundaries, savepoints, the four isolation levels, DDL transactionality.

Patterns & modern SQL

Skill	Covers
sql-gaps-and-islands	The canonical runs-and-gaps pattern, set-based instead of procedural.
sql-temporal-tables	SQL:2011 system-versioned tables and FOR SYSTEM_TIME queries.
sql-match-recognize	SQL:2016 MATCH_RECOGNIZE row-pattern recognition for time-series.
sql-property-graph-queries	SQL:2023 SQL/PGQ — property graphs over relational tables.

Portability

Skill	Covers
sql-standard-vs-dialect-map	The portability index — each standard feature mapped to engine support and spelling.

Layout

.claude-plugin/marketplace.json     marketplace manifest
plugins/sql/
  .claude-plugin/plugin.json        plugin manifest
  skills/<skill-name>/
    SKILL.md                        rule + worked examples
    references/common-mistakes.md   antipattern catalog
reports/                            research, planning, and audit notes