Restrict DEFAULT NULL logic to CREATE TABLE operations only

Modified tableSpecToSchema to only add implicit DEFAULT NULL
when called for CREATE TABLE operations, not for ALTER TABLE
or other schema processing operations. This prevents unintended
side effects on existing table modifications.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: elianddb

sql/planbuilder/ddl.go

@@ -326,7 +326,7 @@ func (b *Builder) buildCreateTable(inScope *scope, c *ast.DDL) (outScope *scope)
 
 	idxDefs := b.buildIndexDefs(inScope, c.TableSpec)
 
-	schema, collation, tblOpts := b.tableSpecToSchema(inScope, outScope, database, strings.ToLower(c.Table.Name.String()), c.TableSpec, false)
+	schema, collation, tblOpts := b.tableSpecToSchemaWithDefaults(inScope, outScope, database, strings.ToLower(c.Table.Name.String()), c.TableSpec, false, true)
 	fkDefs, chDefs := b.buildConstraintsDefs(outScope, c.Table, c.TableSpec)
 
 	schema.Schema = assignColumnIndexesInSchema(schema.Schema)
@@ -442,7 +442,7 @@ func (b *Builder) buildCreateTableLike(inScope *scope, ct *ast.DDL) *scope {
 	var comment string
 	outScope := inScope.push()
 	if ct.TableSpec != nil {
-		pkSch, coll, _ = b.tableSpecToSchema(inScope, outScope, database, strings.ToLower(ct.Table.Name.String()), ct.TableSpec, false)
+		pkSch, coll, _ = b.tableSpecToSchemaWithDefaults(inScope, outScope, database, strings.ToLower(ct.Table.Name.String()), ct.TableSpec, false, true)
 	}
 
 	var pkOrdinals []int
@@ -1335,6 +1335,11 @@ func validateOnUpdateExprs(col *sql.Column) error {
 
 // TableSpecToSchema creates a sql.Schema from a parsed TableSpec and returns the parsed primary key schema, collation ID, and table comment.
 func (b *Builder) tableSpecToSchema(inScope, outScope *scope, db sql.Database, tableName string, tableSpec *ast.TableSpec, forceInvalidCollation bool) (sql.PrimaryKeySchema, sql.CollationID, map[string]interface{}) {
+	return b.tableSpecToSchemaWithDefaults(inScope, outScope, db, tableName, tableSpec, forceInvalidCollation, false)
+}
+
+// tableSpecToSchemaWithDefaults is the internal implementation that allows controlling whether to add implicit DEFAULT NULL
+func (b *Builder) tableSpecToSchemaWithDefaults(inScope, outScope *scope, db sql.Database, tableName string, tableSpec *ast.TableSpec, forceInvalidCollation bool, addImplicitDefaults bool) (sql.PrimaryKeySchema, sql.CollationID, map[string]interface{}) {
 	// TODO: helper method?
 	tblOpts := make(map[string]interface{})
 	for _, tblOpt := range tableSpec.TableOpts {
@@ -1419,7 +1424,7 @@ func (b *Builder) tableSpecToSchema(inScope, outScope *scope, db sql.Database, t
 
 	for i, def := range defaults {
 		schema[i].Default = b.convertDefaultExpression(outScope, def, schema[i].Type, schema[i].Nullable)
-		if schema[i].Default == nil && schema[i].Nullable {
+		if addImplicitDefaults && schema[i].Default == nil && schema[i].Nullable {
 			schema[i].Default = &sql.ColumnDefaultValue{
 				Expr:          b.buildScalar(inScope, &ast.NullVal{}),
 				OutType:       types.Null,