Prevent user and system variables in column default values

Added validation to prevent user variables (@var) and system variables
(@@var) from being used in column default expressions, matching MySQL
behavior that throws Error 3772.

- Added ErrColumnDefaultUserVariable error for user/system variable validation
- Extended validateColumnDefault function to check for UserVar and SystemVar expressions
- Added tests for both user and system variable validation in column defaults

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

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

Author: elianddb

enginetest/queries/column_default_queries.go

@@ -940,4 +940,23 @@ var ColumnDefaultTests = []ScriptTest{
 			},
 		},
 	},
+	{
+		Name: "User variables not allowed in column default expressions",
+		SetUpScript: []string{"SET @test_var = 1"},
+		Assertions: []ScriptTestAssertion{
+			{
+				Query:       "CREATE TABLE t999(pk BIGINT PRIMARY KEY, v1 BIGINT DEFAULT (@test_var))",
+				ExpectedErr: sql.ErrColumnDefaultUserVariable,
+			},
+		},
+	},
+	{
+		Name: "System variables not allowed in column default expressions",
+		Assertions: []ScriptTestAssertion{
+			{
+				Query:       "CREATE TABLE t999(pk BIGINT PRIMARY KEY, v1 VARCHAR(100) DEFAULT (@@session.sql_mode))",
+				ExpectedErr: sql.ErrColumnDefaultUserVariable,
+			},
+		},
+	},
 }

sql/analyzer/resolve_column_defaults.go

@@ -233,6 +233,9 @@ func validateColumnDefault(ctx *sql.Context, col *sql.Column, colDefault *sql.Co
 	var err error
 	sql.Inspect(colDefault.Expr, func(e sql.Expression) bool {
 		switch e.(type) {
+		case *expression.UserVar, *expression.SystemVar:
+			err = sql.ErrColumnDefaultUserVariable.New(col.Name)
+			return false
 		case sql.FunctionExpression, *expression.UnresolvedFunction:
 			var funcName string
 			switch expr := e.(type) {

sql/errors.go

@@ -156,6 +156,9 @@ var (
 	// ErrDropColumnReferencedInDefault is returned when a column cannot be dropped as it is referenced by another column's default value.
 	ErrDropColumnReferencedInDefault = errors.NewKind(`cannot drop column "%s" as default value of column "%s" references it`)
 
+	// ErrColumnDefaultUserVariable is returned when a default/generated column expression contains user or system variables.
+	ErrColumnDefaultUserVariable = errors.NewKind(`Default value expression of column '%s' cannot refer user or system variables.`)
+
 	// ErrTriggersNotSupported is returned when attempting to create a trigger on a database that doesn't support them
 	ErrTriggersNotSupported = errors.NewKind(`database "%s" doesn't support triggers`)