Add PERIOD FOR SYSTEM_TIME support in ALTER TABLE statements

- Handle ADD PERIOD FOR SYSTEM_TIME (start, end) in ALTER TABLE ADD
- Handle DROP PERIOD FOR SYSTEM_TIME in ALTER TABLE DROP
- Set TableElementType to "Period" for period elements

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kyleconroy

parser/parse_ddl.go

@@ -5117,6 +5117,29 @@ func (p *Parser) parseAlterTableDropStatement(tableName *ast.SchemaObjectName) (
 		case p.curTok.Type == TokenIndex:
 			currentElementType = "Index"
 			p.nextToken()
+		case strings.ToUpper(p.curTok.Literal) == "PERIOD":
+			// DROP PERIOD FOR SYSTEM_TIME
+			currentElementType = "Period"
+			p.nextToken() // consume PERIOD
+			if strings.ToUpper(p.curTok.Literal) == "FOR" {
+				p.nextToken() // consume FOR
+			}
+			if strings.ToUpper(p.curTok.Literal) == "SYSTEM_TIME" {
+				p.nextToken() // consume SYSTEM_TIME
+			}
+			// Create the element with no name
+			element := &ast.AlterTableDropTableElement{
+				TableElementType: currentElementType,
+				IsIfExists:       false,
+			}
+			stmt.AlterTableDropTableElements = append(stmt.AlterTableDropTableElements, element)
+			// Reset and continue
+			currentElementType = "NotSpecified"
+			if p.curTok.Type == TokenComma {
+				p.nextToken() // consume comma
+				continue
+			}
+			break
 		}
 
 		// Check for IF EXISTS
@@ -6579,6 +6602,32 @@ func (p *Parser) parseAlterTableAddStatement(tableName *ast.SchemaObjectName) (*
 					stmt.Definition.TableConstraints = append(stmt.Definition.TableConstraints, constraint)
 				}
 			}
+		} else if strings.ToUpper(p.curTok.Literal) == "PERIOD" {
+			// Parse PERIOD FOR SYSTEM_TIME (start, end)
+			p.nextToken() // consume PERIOD
+			if strings.ToUpper(p.curTok.Literal) == "FOR" {
+				p.nextToken() // consume FOR
+			}
+			if strings.ToUpper(p.curTok.Literal) == "SYSTEM_TIME" {
+				p.nextToken() // consume SYSTEM_TIME
+			}
+			// Parse (start_column, end_column)
+			var startCol, endCol *ast.Identifier
+			if p.curTok.Type == TokenLParen {
+				p.nextToken() // consume (
+				startCol = p.parseIdentifier()
+				if p.curTok.Type == TokenComma {
+					p.nextToken() // consume ,
+				}
+				endCol = p.parseIdentifier()
+				if p.curTok.Type == TokenRParen {
+					p.nextToken() // consume )
+				}
+			}
+			stmt.Definition.SystemTimePeriod = &ast.SystemTimePeriodDefinition{
+				StartTimeColumn: startCol,
+				EndTimeColumn:   endCol,
+			}
 		} else {
 			// Parse column definition (column_name data_type ...)
 			colDef, err := p.parseColumnDefinition()

parser/testdata/AlterTableStatementTests130/metadata.json

@@ -1 +1 @@
-{"todo": true}
+{}

parser/testdata/Baselines130_AlterTableStatementTests130/metadata.json

@@ -1 +1 @@
-{"todo": true}
+{}