Fix single-table aliases and empty schema tables

AGENTS.md

@@ -72,7 +72,7 @@ Quick examples:
 - New chase variants should be composable with existing infrastructure.
 - Existential variables generate labeled nulls (`Term::Null`).
 - The current SQL support is intentionally narrow: `SELECT-FROM-WHERE-ORDER BY` over predicate-backed tables, equality predicates combined with `AND`, comma-join style multi-table queries, table aliases, and ordering by output-column names.
-- Stable SQL column names come from explicit catalog registration or the frontend `schema ...` command; otherwise the default names are positional such as `c0` and `c1`.
+- Stable SQL column names come from explicit catalog registration or the frontend `schema ...` command, including for empty tables; otherwise the default names are positional such as `c0` and `c1`.
 - Do not describe unsupported SQL features such as aggregates, grouping, or arbitrary expressions as implemented.
 - Relational and SQL modules should build on explicit schemas and logical plans, not call frontend helpers directly.
 - If you add parser, planner, or executor layers, keep their responsibilities separate.

README.md

@@ -137,8 +137,12 @@ In the REPL or script runner, use the `sql` command and end the statement with
 sql SELECT c0 FROM Parent WHERE c1 = 'bob';
 ```
 
+`fact`, `rule`, `schema`, `sql`, `query`, and `explain` commands may also span
+multiple lines in `.ech` scripts as long as the final line ends with the normal
+terminator.
+
 You can also register stable column names for a predicate-backed table in the
-frontend before running SQL:
+frontend before running SQL, including tables that currently have no facts:
 
 ```text
 schema Parent(parent, child).

src/frontend/session.rs

@@ -3,13 +3,13 @@
 use std::collections::HashMap;
 use std::fmt;
 
-use crate::catalog::PredicateCatalog;
+use crate::catalog::{CatalogError, PredicateCatalog};
 use crate::chase::{
     Atom, Instance, MaterializedState, Rule, Substitution, find_matches, materialize,
 };
 use crate::execution::execute;
 use crate::planner::sql::plan_select;
-use crate::relational::ResultSet;
+use crate::relational::{DataType, Field, ResultSet, Schema};
 
 use super::language::{Command, parse_script};
 use super::provenance::explain_atom;
@@ -143,9 +143,24 @@ impl Session {
         let mut catalog =
             PredicateCatalog::from_instance(instance).map_err(|err| err.to_string())?;
         for (table, columns) in &self.column_names {
-            catalog
-                .rename_columns(table, columns.clone())
-                .map_err(|err| err.to_string())?;
+            match catalog.schema_for(table) {
+                Ok(_) => {
+                    catalog
+                        .rename_columns(table, columns.clone())
+                        .map_err(|err| err.to_string())?;
+                }
+                Err(CatalogError::UnknownTable(_)) => {
+                    let schema = Schema::new(
+                        columns
+                            .iter()
+                            .cloned()
+                            .map(|name| Field::new(name, DataType::Text, false))
+                            .collect(),
+                    );
+                    catalog.register_table(table.clone(), schema);
+                }
+                Err(err) => return Err(err.to_string()),
+            }
         }
         let plan = plan_select(select, &catalog).map_err(|err| err.to_string())?;
         let result = execute(&plan, instance).map_err(|err| err.to_string())?;
@@ -401,6 +416,21 @@ mod tests {
         assert!(output.contains("alice"));
     }
 
+    #[test]
+    fn session_runs_sql_query_with_declared_empty_table() {
+        let mut session = Session::new();
+        let output = session
+            .execute_script(
+                "schema Parent(parent, child).\n\
+                 sql SELECT parent FROM Parent;",
+            )
+            .unwrap();
+
+        assert!(output.contains("Registered schema for Parent: parent, child"));
+        assert!(output.contains("0 row(s)"));
+        assert!(output.contains("parent"));
+    }
+
     #[test]
     fn session_runs_sql_join_query() {
         let mut session = Session::new();
@@ -443,6 +473,22 @@ mod tests {
         assert!(output.contains("bob | dave"));
     }
 
+    #[test]
+    fn session_runs_sql_query_with_single_table_alias() {
+        let mut session = Session::new();
+        let output = session
+            .execute_script(
+                "fact Parent(alice, bob).\n\
+                 schema Parent(parent, child).\n\
+                 sql SELECT p.parent FROM Parent AS p WHERE p.child = 'bob';",
+            )
+            .unwrap();
+
+        assert!(output.contains("1 row(s)"));
+        assert!(output.contains("p.parent"));
+        assert!(output.contains("alice"));
+    }
+
     #[test]
     fn session_runs_sql_query_with_and_filter() {
         let mut session = Session::new();

src/planner/sql.rs

@@ -125,7 +125,8 @@ fn plan_from_tables(
         return Err(PlannerError::DuplicateSourceName(first_name));
     }
 
-    let first_schema = input_schema_for_table(first, catalog, tables.len() > 1)?;
+    let first_schema =
+        input_schema_for_table(first, catalog, should_qualify_columns(first, tables))?;
     let mut plan = LogicalPlan::Scan {
         table: first.name.clone(),
         schema: first_schema.clone(),
@@ -138,7 +139,8 @@ fn plan_from_tables(
             return Err(PlannerError::DuplicateSourceName(qualified_name));
         }
 
-        let right_schema = input_schema_for_table(table, catalog, tables.len() > 1)?;
+        let right_schema =
+            input_schema_for_table(table, catalog, should_qualify_columns(table, tables))?;
         let join_schema = combine_schemas(&combined_schema, &right_schema);
         let right_plan = LogicalPlan::Scan {
             table: table.name.clone(),
@@ -266,6 +268,10 @@ fn input_schema_for_table(
     Ok(Schema::new(fields))
 }
 
+fn should_qualify_columns(table: &TableRef, tables: &[TableRef]) -> bool {
+    table.alias.is_some() || tables.len() > 1
+}
+
 fn source_name(table: &TableRef) -> String {
     table.alias.clone().unwrap_or_else(|| table.name.clone())
 }
@@ -386,6 +392,28 @@ mod tests {
         assert_eq!(schema.fields()[1].name(), "q.child");
     }
 
+    #[test]
+    fn plans_single_table_with_alias() {
+        let instance: Instance = vec![Atom::new(
+            "Parent",
+            vec![Term::constant("alice"), Term::constant("bob")],
+        )]
+        .into_iter()
+        .collect();
+        let mut catalog = PredicateCatalog::from_instance(&instance).unwrap();
+        catalog
+            .rename_columns("Parent", ["parent", "child"])
+            .unwrap();
+
+        let select =
+            parse_select("SELECT p.parent FROM Parent AS p WHERE p.child = 'bob'").unwrap();
+
+        let plan = plan_select(&select, &catalog).unwrap();
+        let schema = plan.output_schema();
+        assert_eq!(schema.len(), 1);
+        assert_eq!(schema.fields()[0].name(), "p.parent");
+    }
+
     #[test]
     fn plans_conjunctive_filter() {
         let instance: Instance = vec![Atom::new(