Add UNDROP TABLE statement support

Adds support for parsing and explaining UNDROP TABLE statements:
- UNDROP TABLE name
- UNDROP TABLE db.name
- UNDROP TABLE name ON CLUSTER cluster
- UNDROP TABLE name UUID 'uuid'

Fixes 7 statements in 02681_undrop_query test (16→9 pending).

Author: claude

ast/ast.go

@@ -499,6 +499,19 @@ func (d *DropQuery) Pos() token.Position { return d.Position }
 func (d *DropQuery) End() token.Position { return d.Position }
 func (d *DropQuery) statementNode()      {}
 
+// UndropQuery represents an UNDROP TABLE statement.
+type UndropQuery struct {
+	Position  token.Position `json:"-"`
+	Database  string         `json:"database,omitempty"`
+	Table     string         `json:"table"`
+	OnCluster string         `json:"on_cluster,omitempty"`
+	UUID      string         `json:"uuid,omitempty"`
+}
+
+func (u *UndropQuery) Pos() token.Position { return u.Position }
+func (u *UndropQuery) End() token.Position { return u.Position }
+func (u *UndropQuery) statementNode()      {}
+
 // AlterQuery represents an ALTER statement.
 type AlterQuery struct {
 	Position  token.Position  `json:"-"`

internal/explain/explain.go

@@ -113,6 +113,8 @@ func Node(sb *strings.Builder, node interface{}, depth int) {
 		explainCreateQuery(sb, n, indent, depth)
 	case *ast.DropQuery:
 		explainDropQuery(sb, n, indent, depth)
+	case *ast.UndropQuery:
+		explainUndropQuery(sb, n, indent, depth)
 	case *ast.RenameQuery:
 		explainRenameQuery(sb, n, indent, depth)
 	case *ast.ExchangeQuery:

internal/explain/statements.go

@@ -505,6 +505,21 @@ func explainDropQuery(sb *strings.Builder, n *ast.DropQuery, indent string, dept
 	}
 }
 
+func explainUndropQuery(sb *strings.Builder, n *ast.UndropQuery, indent string, depth int) {
+	name := n.Table
+	// Check if we have a database-qualified name (for UNDROP TABLE db.table)
+	hasDatabase := n.Database != ""
+	if hasDatabase {
+		// Database-qualified: UndropQuery db table (children 2)
+		fmt.Fprintf(sb, "%sUndropQuery %s %s (children %d)\n", indent, EscapeIdentifier(n.Database), EscapeIdentifier(name), 2)
+		fmt.Fprintf(sb, "%s Identifier %s\n", indent, EscapeIdentifier(n.Database))
+		fmt.Fprintf(sb, "%s Identifier %s\n", indent, EscapeIdentifier(name))
+	} else {
+		fmt.Fprintf(sb, "%sUndropQuery  %s (children %d)\n", indent, EscapeIdentifier(name), 1)
+		fmt.Fprintf(sb, "%s Identifier %s\n", indent, EscapeIdentifier(name))
+	}
+}
+
 func explainRenameQuery(sb *strings.Builder, n *ast.RenameQuery, indent string, depth int) {
 	if n == nil {
 		fmt.Fprintf(sb, "%s*ast.RenameQuery\n", indent)

parser/parser.go

@@ -180,6 +180,8 @@ func (p *Parser) parseStatement() ast.Statement {
 		return p.parseAlter()
 	case token.TRUNCATE:
 		return p.parseTruncate()
+	case token.UNDROP:
+		return p.parseUndrop()
 	case token.USE:
 		return p.parseUse()
 	case token.DESCRIBE, token.DESC:
@@ -4446,6 +4448,50 @@ func (p *Parser) parseTruncate() *ast.TruncateQuery {
 	return trunc
 }
 
+func (p *Parser) parseUndrop() *ast.UndropQuery {
+	undrop := &ast.UndropQuery{
+		Position: p.current.Pos,
+	}
+
+	p.nextToken() // skip UNDROP
+
+	if p.currentIs(token.TABLE) {
+		p.nextToken()
+	}
+
+	// Parse table name (can start with a number in ClickHouse)
+	tableName := p.parseIdentifierName()
+	if tableName != "" {
+		if p.currentIs(token.DOT) {
+			p.nextToken()
+			undrop.Database = tableName
+			undrop.Table = p.parseIdentifierName()
+		} else {
+			undrop.Table = tableName
+		}
+	}
+
+	// Handle ON CLUSTER
+	if p.currentIs(token.ON) {
+		p.nextToken()
+		if p.currentIs(token.CLUSTER) {
+			p.nextToken()
+			undrop.OnCluster = p.parseIdentifierName()
+		}
+	}
+
+	// Handle UUID
+	if p.currentIs(token.IDENT) && strings.ToUpper(p.current.Value) == "UUID" {
+		p.nextToken()
+		if p.currentIs(token.STRING) {
+			undrop.UUID = p.current.Value
+			p.nextToken()
+		}
+	}
+
+	return undrop
+}
+
 func (p *Parser) parseUse() *ast.UseQuery {
 	use := &ast.UseQuery{
 		Position: p.current.Pos,

parser/testdata/02681_undrop_query/metadata.json

@@ -1,6 +1,5 @@
 {
   "explain_todo": {
-    "stmt16": true,
     "stmt22": true,
     "stmt23": true,
     "stmt25": true,
@@ -9,12 +8,6 @@
     "stmt32": true,
     "stmt34": true,
     "stmt36": true,
-    "stmt38": true,
-    "stmt45": true,
-    "stmt54": true,
-    "stmt62": true,
-    "stmt76": true,
-    "stmt78": true,
-    "stmt8": true
+    "stmt38": true
   }
 }

token/token.go

@@ -191,6 +191,7 @@ const (
 	TRUE
 	TRUNCATE
 	TTL
+	UNDROP
 	UNION
 	UPDATE
 	USE
@@ -387,6 +388,7 @@ var tokens = [...]string{
 	TRUE:         "TRUE",
 	TRUNCATE:     "TRUNCATE",
 	TTL:          "TTL",
+	UNDROP:       "UNDROP",
 	UNION:        "UNION",
 	UPDATE:       "UPDATE",
 	USE:          "USE",