web: add SQL-entry path to the new-module wizard

The wizard previously required picking a single source table; modules whose
entry point is arbitrary SQL (CTEs, joins, computed columns) didn't fit. Add a
"write SQL" path alongside "browse a table":

- Driver.introspect_query_columns + _zero_row_wrap discover a query's result
  columns by running it with ~no rows. Generic wrap is a derived table with
  WHERE 1=0; DB2 appends FETCH FIRST 1 ROW ONLY (DB2 for i forbids WITH inside
  a nested table expression).
- /wizard/sql + POST /wizard/sql/columns seed the column-mapping grid; dest
  types default to text (no result-set type metadata over jrunner CSV).
- wizard_step3.html grows a sql_mode branch (array-named inputs, query shown
  verbatim, no column unchecking); wizard_create branches on entry_mode.

Verified end-to-end against a live DB2 for i connection, including a top-level
CTE query.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CLAUDE.md

@@ -100,6 +100,10 @@ Passwords in connections are stored as `$VAR_NAME` references. At run time they
 
 Each driver in `pipekit/drivers/` inherits from `base.py::Driver` and implements: `browse_fields`, `list_tables`, `list_schemas`, `get_columns`, `map_type`, `default_expression`, `quote_identifier`. The wizard UI calls `/api/introspect/*` which dispatches to the appropriate driver.
 
+## Wizard Entry Modes
+
+Step 1 offers two paths. **Browse a table** (`/wizard/tables` → `/wizard/columns`) is the original flow: pick one source table, introspect its columns. **Write SQL** (`/wizard/sql` → `POST /wizard/sql/columns`) starts from an arbitrary query — the source query is used verbatim and its result columns are discovered by `Driver.introspect_query_columns`, which runs the query wrapped to fetch ~no rows (`_zero_row_wrap`: derived-table + `WHERE 1=0` by default; DB2 appends `FETCH FIRST 1 ROW ONLY` since DB2 for i forbids `WITH` inside a nested table expression). Both paths land on `wizard_step3.html` (the SQL path passes `sql_mode=True`) and POST to `/wizard/create`, which branches on `entry_mode`. SQL-mode dest types default to `text` — there's no result-set type metadata over jrunner's CSV output, so the user adjusts types by hand. `columns_json` is never read at run time (the engine does `CREATE staging LIKE dest` + `SELECT *`); it only drives the dest `CREATE TABLE`.
+
 ## Module Columns
 
 Modules store their column mapping as `columns_json` — a JSON list of dicts with keys `source_name`, `source_type`, `dest_name`, `dest_type`. The engine uses this to build the staging CREATE TABLE and the merge INSERT column lists.

pipekit/drivers/base.py

@@ -181,6 +181,24 @@ class Driver(abc.ABC):
             return None
         return {row[0].strip().lower() for row in r.rows if row and row[0]}
 
+    # ---- Arbitrary-query introspection (SQL-entry wizard path) ----
+    def introspect_query_columns(self, conn: dict, sql: str) -> list[str]:
+        """Return the result-set column names of an arbitrary user query,
+        fetching (near) zero rows. Used by the wizard's SQL-entry path to
+        seed the column-mapping grid when the source isn't a single table."""
+        wrapped = self._zero_row_wrap(sql)
+        r = self.query(conn, wrapped)
+        return [(c or "").strip() for c in r.columns]
+
+    def _zero_row_wrap(self, sql: str) -> str:
+        """Wrap an arbitrary query so it yields its columns but ~no rows.
+        Generic default wraps it as a derived table with a false predicate;
+        this works on dialects where a sub-SELECT may carry its own WITH
+        (e.g. PostgreSQL). Dialects that forbid WITH inside a nested table
+        expression (DB2 for i, MSSQL) override this."""
+        body = sql.strip().rstrip(";").rstrip()
+        return f"SELECT * FROM (\n{body}\n) AS _pk_introspect WHERE 1=0"
+
     # ---- Shared helper ----
     def query(self, conn: dict, sql: str) -> jrunner.QueryResult:
         """Run `sql` in jrunner query mode against `conn`."""

pipekit/drivers/db2.py

@@ -115,6 +115,13 @@ class DB2Driver(Driver):
         v = result.rows[0][0].strip()
         return v or None
 
+    def _zero_row_wrap(self, sql: str) -> str:
+        # DB2 for i forbids a WITH clause inside a nested table expression,
+        # so we can't wrap the query as a derived table. Append a fetch-limit
+        # to the outer statement instead. (0 is rejected — must be positive.)
+        body = sql.strip().rstrip(";").rstrip()
+        return f"{body}\nFETCH FIRST 1 ROW ONLY"
+
     def qualified_table_name(self, table: str, *, schema: str) -> str:
         return f"{self.quote_identifier(schema)}.{self.quote_identifier(table)}"
 

pipekit/web/app.py

@@ -541,15 +541,106 @@ def wizard_step3(request: Request,
     )
 
 
+@_router.get("/wizard/sql", response_class=HTMLResponse)
+def wizard_sql_entry(request: Request,
+                     source_connection_id: int = Query(...)):
+    """Alternate step 2 — write an arbitrary source query instead of
+    browsing a single table."""
+    conn = repo.get_connection(source_connection_id)
+    if conn is None:
+        raise HTTPException(404, f"connection id={source_connection_id} not found")
+    drv = _driver_for_conn(conn)
+    if drv is None:
+        raise HTTPException(500, "driver row missing for connection")
+    return _templates.TemplateResponse(
+        request,
+        "wizard_sql.html",
+        _ctx(step=2, mode="sql", connection=conn, driver_kind=drv.kind,
+             source_query=""),
+    )
+
+
+@_router.post("/wizard/sql/columns", response_class=HTMLResponse)
+async def wizard_sql_columns(request: Request):
+    """Introspect an arbitrary query's result columns and render the
+    column-mapping grid (SQL-entry variant of step 3)."""
+    form = await request.form()
+    source_connection_id = int(form["source_connection_id"])
+    source_query = (form.get("source_query") or "").strip()
+
+    conn = repo.get_connection(source_connection_id)
+    if conn is None:
+        raise HTTPException(404, f"connection id={source_connection_id} not found")
+    drv = _driver_for_conn(conn)
+    if drv is None:
+        raise HTTPException(500, "driver row missing for connection")
+
+    if not source_query:
+        return _templates.TemplateResponse(
+            request, "wizard_sql.html",
+            _ctx(step=2, mode="sql", connection=conn, driver_kind=drv.kind,
+                 source_query=source_query,
+                 fetch_error="Enter a source query first."),
+        )
+
+    fetch_error: str | None = None
+    columns: list[dict] = []
+    try:
+        names = drv.introspect_query_columns(conn, source_query)
+        for i, name in enumerate(names):
+            columns.append({
+                "position": i + 1,
+                "name": name,
+                "default_dest_name": _sanitize_identifier(name),
+                "default_dest_type": "text",
+                "default_description": "",
+            })
+    except (jrunner.JrunnerError, ValueError) as e:
+        fetch_error = str(e)
+    except Exception as e:                                           # noqa: BLE001
+        fetch_error = f"{type(e).__name__}: {e}"
+
+    if fetch_error or not columns:
+        return _templates.TemplateResponse(
+            request, "wizard_sql.html",
+            _ctx(step=2, mode="sql", connection=conn, driver_kind=drv.kind,
+                 source_query=source_query,
+                 fetch_error=fetch_error or "Query returned no columns."),
+        )
+
+    drivers_by_id = {d["id"]: d for d in repo.list_drivers()}
+    dest_conns = [
+        c for c in repo.list_connections()
+        if drivers_by_id.get(c["driver_id"]) is not None
+    ]
+    default_dest_conn_id = conn.get("default_dest_connection_id")
+    default_dest_schema = conn.get("default_dest_schema") or ""
+
+    return _templates.TemplateResponse(
+        request,
+        "wizard_step3.html",
+        _ctx(step=3, mode="sql", sql_mode=True, connection=conn,
+             all_connections=dest_conns, driver_kind=drv.kind,
+             columns=columns, source_query=source_query,
+             qualified_table="(custom query)", table="", qvals={},
+             table_description="", fetch_error=None,
+             default_module_name="",
+             default_dest_conn_id=default_dest_conn_id,
+             default_dest_schema=default_dest_schema,
+             dest_warn=None),
+    )
+
+
 @_router.post("/wizard/create")
 async def wizard_create(request: Request):
     """Step 4 — build source_query from picks, create the module,
     and provision the destination schema + table."""
     form = await request.form()
 
+    entry_mode = form.get("entry_mode", "table")
     source_connection_id = int(form["source_connection_id"])
     dest_connection_id = int(form["dest_connection_id"])
-    table = form["table"]
+    table = form.get("table", "")
     module_name = form["module_name"].strip()
     dest_table = form["dest_table"].strip()
     merge_strategy = form.get("merge_strategy", "full")
@@ -576,6 +667,34 @@ async def wizard_create(request: Request):
     if dest_drv is None:
         raise HTTPException(500, "driver row missing for dest connection")
 
+    if entry_mode == "sql":
+        # SQL-entry path: the source query is used verbatim and the column
+        # mapping comes from parallel arrays seeded by query introspection.
+        source_query = (form.get("source_query") or "").strip()
+        if not source_query:
+            raise HTTPException(400, "source query is required")
+        col_names = form.getlist("sql_col_name")
+        dest_names = form.getlist("sql_dest_name")
+        dest_types = form.getlist("sql_dest_type")
+        dest_descs = form.getlist("sql_dest_desc")
+        chosen = []
+        for i, src_name in enumerate(col_names):
+            dest_name = (dest_names[i] if i < len(dest_names) else "").strip()
+            dest_type = (dest_types[i] if i < len(dest_types) else "").strip()
+            desc = (dest_descs[i] if i < len(dest_descs) else "").strip() or None
+            if not dest_name or not dest_type:
+                raise HTTPException(
+                    400, f"column {src_name!r} missing dest_name or dest_type")
+            chosen.append({
+                "source_name": src_name,
+                "source_type": "",
+                "dest_name": dest_name,
+                "dest_type": dest_type,
+                "description": desc,
+            })
+        if not chosen:
+            raise HTTPException(400, "no columns to create")
+    else:
         qvals: dict = {}
         for f in src_drv.browse_fields():
             v = form.get(f.name)
@@ -636,11 +755,15 @@ async def wizard_create(request: Request):
         missing = [c["dest_name"] for c in chosen
                    if c["dest_name"].lower() not in existing_cols]
         if missing:
+            if entry_mode == "sql":
+                back_url = f"/wizard/sql?source_connection_id={source_connection_id}"
+            else:
                 back_qs = urlencode(
                     [("source_connection_id", source_connection_id),
                      ("table", table),
                      ("table_schema", qvals.get("schema") or qvals.get("library") or ""),
                      *qvals.items()])
+                back_url = f"/wizard/columns?{back_qs}"
             return _templates.TemplateResponse(
                 request,
                 "wizard_error.html",
@@ -649,7 +772,7 @@ async def wizard_create(request: Request):
                     qualified_dest=qualified_dest,
                     missing=missing,
                     existing=sorted(existing_cols),
-                    back_qs=back_qs,
+                    back_url=back_url,
                 ),
                 status_code=409,
             )

pipekit/web/templates/_wizard_steps.html

@@ -4,7 +4,7 @@
         <span class="num">1</span> source connection
     </div>
     <div class="step {% if step == 2 %}active{% elif step > 2 %}done{% endif %}">
-        <span class="num">2</span> browse tables
+        <span class="num">2</span> {% if mode == "sql" %}write SQL{% else %}browse tables{% endif %}
     </div>
     <div class="step {% if step == 3 %}active{% elif step > 3 %}done{% endif %}">
         <span class="num">3</span> columns &amp; config

pipekit/web/templates/wizard_error.html

@@ -8,7 +8,7 @@
         Wizard error
         <span class="subtitle">{{ title }}</span>
         <span style="margin-left:auto">
-            <a href="/wizard/columns?{{ back_qs }}">&larr; back to step 3</a>
+            <a href="{{ back_url }}">&larr; back to step 3</a>
         </span>
     </header>
     <div class="body">

pipekit/web/templates/wizard_sql.html

@@ -0,0 +1,43 @@
+{% extends "base.html" %}
+{% set section = "modules" %}
+{% block title %}New module — write SQL{% endblock %}
+
+{% block content %}
+{% include "_wizard_steps.html" %}
+
+<div class="panel">
+    <header>
+        Step 2 — write the source query
+        <span class="subtitle">{{ connection.name }} ({{ driver_kind }})</span>
+        <span style="margin-left:auto"><a href="/wizard">&larr; different connection</a></span>
+    </header>
+    <div class="body">
+        {% if fetch_error %}
+        <div class="flash err" style="margin-bottom:0.8rem">
+            Could not introspect the query's columns:
+            <pre class="sql" style="margin-top:0.4rem">{{ fetch_error }}</pre>
+        </div>
+        {% endif %}
+
+        <form method="post" action="/wizard/sql/columns">
+            <input type="hidden" name="source_connection_id" value="{{ connection.id }}">
+            <label class="field">
+                <span>source query</span>
+                <textarea name="source_query" id="source-query" rows="18" required
+                          class="mono" style="width:100%"
+                          placeholder="SELECT ... FROM ...">{{ source_query }}</textarea>
+                <span class="help">
+                    Runs verbatim against the source at run time. Use
+                    <code>{name}</code> placeholders for watermarks. On
+                    <em>introspect</em>, pipekit runs the query fetching ~no rows to
+                    discover its result columns.
+                </span>
+            </label>
+            <div style="display:flex;justify-content:flex-end;gap:0.5rem;margin-top:0.8rem">
+                <a class="btn ghost" href="/wizard">cancel</a>
+                <button type="submit" class="primary">introspect columns &rarr;</button>
+            </div>
+        </form>
+    </div>
+</div>
+{% endblock %}

pipekit/web/templates/wizard_step1.html

@@ -36,10 +36,15 @@
             {% endfor %}
             </tbody>
         </table>
-        <div class="body" style="display:flex;justify-content:flex-end;gap:0.5rem">
+        <div class="body" style="display:flex;justify-content:flex-end;gap:0.5rem;align-items:center">
             <a class="btn ghost" href="/">cancel</a>
-            <button type="submit" class="primary">next &rarr;</button>
+            <button type="submit" class="btn" formaction="/wizard/sql">write SQL &rarr;</button>
+            <button type="submit" class="primary" formaction="/wizard/tables">browse a table &rarr;</button>
         </div>
+        <p class="help" style="text-align:right;margin:0 0 0.6rem">
+            <strong>browse a table</strong> picks one table and its columns ·
+            <strong>write SQL</strong> starts from an arbitrary query
+        </p>
         </form>
         {% else %}
         <div class="empty">

pipekit/web/templates/wizard_step3.html

@@ -7,9 +7,15 @@
 
 <div class="panel">
     <header>
-        Step 3 — choose columns &amp; configure merge
+        Step 3 — {% if sql_mode %}map columns &amp; configure merge{% else %}choose columns &amp; configure merge{% endif %}
         <span class="subtitle">{{ qualified_table }}</span>
-        <span style="margin-left:auto"><a href="/wizard/tables?source_connection_id={{ connection.id }}{% for k,v in qvals.items() %}&amp;{{ k }}={{ v }}{% endfor %}&amp;browse=1">&larr; different table</a></span>
+        <span style="margin-left:auto">
+            {% if sql_mode %}
+            <a href="/wizard/sql?source_connection_id={{ connection.id }}">&larr; edit SQL</a>
+            {% else %}
+            <a href="/wizard/tables?source_connection_id={{ connection.id }}{% for k,v in qvals.items() %}&amp;{{ k }}={{ v }}{% endfor %}&amp;browse=1">&larr; different table</a>
+            {% endif %}
+        </span>
     </header>
     <div class="body">
         {% if fetch_error %}
@@ -31,36 +37,75 @@
 {% if not fetch_error %}
 <form method="post" action="/wizard/create">
     <input type="hidden" name="source_connection_id" value="{{ connection.id }}">
+    {% if sql_mode %}
+    <input type="hidden" name="entry_mode" value="sql">
+    {% else %}
     <input type="hidden" name="table" value="{{ table }}">
     {% for k, v in qvals.items() %}
         <input type="hidden" name="{{ k }}" value="{{ v }}">
     {% endfor %}
+    {% endif %}
 
     <div class="two-col">
         <div class="panel">
             <header>
                 Columns
+                {% if sql_mode %}
+                <span class="subtitle">{{ columns|length }} from the query — all created; types default to <code>text</code></span>
+                {% else %}
                 <span class="subtitle">{{ columns|length }} total — uncheck to exclude</span>
                 <span style="margin-left:auto">
                     <button type="button" class="ghost" onclick="toggleAll(true)">all</button>
                     <button type="button" class="ghost" onclick="toggleAll(false)">none</button>
                 </span>
+                {% endif %}
             </header>
             <div class="body tight">
                 <table class="grid picker">
                     <thead>
                         <tr>
-                            <th class="pick"></th>
+                            {% if not sql_mode %}<th class="pick"></th>{% endif %}
                             <th style="width:3em">#</th>
                             <th>source name</th>
+                            {% if not sql_mode %}
                             <th>source type</th>
                             <th style="width:3em">null?</th>
+                            {% endif %}
                             <th>dest name</th>
                             <th>dest type</th>
                             <th>description</th>
                         </tr>
                     </thead>
                     <tbody>
+                    {% if sql_mode %}
+                    {% for c in columns %}
+                        <tr>
+                            <td class="mono">{{ c.position }}</td>
+                            <td class="mono">
+                                {{ c.name }}
+                                <input type="hidden" name="sql_col_name" value="{{ c.name }}">
+                            </td>
+                            <td>
+                                <input type="text" class="mono"
+                                       name="sql_dest_name"
+                                       value="{{ c.default_dest_name }}"
+                                       style="width:100%;font-size:12px">
+                            </td>
+                            <td>
+                                <input type="text" class="mono"
+                                       name="sql_dest_type"
+                                       value="{{ c.default_dest_type }}"
+                                       style="width:100%;font-size:12px">
+                            </td>
+                            <td>
+                                <input type="text"
+                                       name="sql_dest_desc"
+                                       value="{{ c.default_description }}"
+                                       style="width:100%;font-size:12px">
+                            </td>
+                        </tr>
+                    {% endfor %}
+                    {% else %}
                     {% for c in columns %}
                         <tr onclick="var cb=document.getElementById('col-{{ loop.index }}'); if(event.target.tagName!=='INPUT') cb.checked=!cb.checked; fillQuery()">
                             <td class="pick">
@@ -92,6 +137,7 @@
                             </td>
                         </tr>
                     {% endfor %}
+                    {% endif %}
                     </tbody>
                 </table>
             </div>
@@ -192,16 +238,24 @@
                 </div>
             </div>
 
-            <div class="panel" id="query-panel" style="display:none">
+            <div class="panel" id="query-panel" style="{% if not sql_mode %}display:none{% endif %}">
                 <header>Source query
+                    {% if sql_mode %}
+                    <span class="subtitle">runs verbatim at run time</span>
+                    {% else %}
                     <span class="subtitle">auto-generated from picks — edit to add WHERE clause</span>
+                    {% endif %}
                 </header>
                 <div class="body">
                     <textarea name="source_query" id="source-query" rows="10" class="mono"
                               style="width:100%" oninput="checkPlaceholders()"
-                              placeholder="Leave blank to auto-generate from column picks"></textarea>
+                              {% if not sql_mode %}placeholder="Leave blank to auto-generate from column picks"{% endif %}>{% if sql_mode %}{{ source_query }}{% endif %}</textarea>
                     <div id="placeholder-warning" class="flash warn" style="display:none;margin-top:0.4rem"></div>
+                    {% if sql_mode %}
+                    <span class="help">Edit if needed. <code>{name}</code> placeholders resolve from watermarks at run time.</span>
+                    {% else %}
                     <span class="help">Leave blank to auto-generate. Add <code>WHERE col &gt; {name}</code> for incremental filtering.</span>
+                    {% endif %}
                 </div>
             </div>
 
@@ -241,6 +295,7 @@
 
 <script>
     const QUALIFIED_TABLE = {{ qualified_table | tojson }};
+    const SQL_MODE = {{ 'true' if sql_mode else 'false' }};
 
     function toggleAll(val) {
         document.querySelectorAll('.col-check').forEach(function (cb) { cb.checked = val; });
@@ -250,12 +305,15 @@
     function onStrategyChange(val) {
         document.getElementById('mkf').style.display = val === 'incremental' ? '' : 'none';
         document.getElementById('wm-panel').style.display = val === 'incremental' ? '' : 'none';
-        document.getElementById('query-panel').style.display = val === 'incremental' ? '' : 'none';
+        // In SQL mode the query panel is always shown (it holds the user's query).
-        if (val === 'incremental') fillQuery();
+        document.getElementById('query-panel').style.display =
+            (SQL_MODE || val === 'incremental') ? '' : 'none';
+        if (!SQL_MODE && val === 'incremental') fillQuery();
         checkPlaceholders();
     }
 
     function fillQuery() {
+        if (SQL_MODE) return;   // never overwrite a hand-written query
         const ta = document.getElementById('source-query');
         if (!ta) return;
         const checked = [...document.querySelectorAll('.col-check:checked')];