🐛 fix(table-designer): 修复金仓新增字段保存失败

Fixes #305

frontend/src/components/tableDesignerSchemaSql.test.ts

@@ -0,0 +1,54 @@
+import { describe, expect, it } from 'vitest';
+
+import {
+  buildAlterTablePreviewSql,
+  type BuildAlterTablePreviewInput,
+  type EditableColumnSnapshot,
+} from './tableDesignerSchemaSql';
+
+const baseColumn = (overrides: Partial<EditableColumnSnapshot>): EditableColumnSnapshot => ({
+  _key: overrides._key || 'col',
+  name: overrides.name || 'id',
+  type: overrides.type || 'int',
+  nullable: overrides.nullable || 'NO',
+  default: overrides.default || '',
+  extra: overrides.extra || '',
+  comment: overrides.comment || '',
+  key: overrides.key || '',
+  isAutoIncrement: overrides.isAutoIncrement || false,
+});
+
+const buildInput = (overrides: Partial<BuildAlterTablePreviewInput>): BuildAlterTablePreviewInput => ({
+  dbType: overrides.dbType || 'mysql',
+  tableName: overrides.tableName || 'users',
+  originalColumns: overrides.originalColumns || [baseColumn({ _key: 'id', name: 'id', key: 'PRI', nullable: 'NO' })],
+  columns: overrides.columns || [
+    baseColumn({ _key: 'id', name: 'id', key: 'PRI', nullable: 'NO' }),
+    baseColumn({ _key: 'age', name: 'age', nullable: 'YES', comment: '年龄' }),
+  ],
+});
+
+describe('tableDesignerSchemaSql', () => {
+  it('keeps mysql alter preview syntax with column position clauses', () => {
+    const sql = buildAlterTablePreviewSql(buildInput({ dbType: 'mysql' }));
+
+    expect(sql).toContain('ALTER TABLE `users`');
+    expect(sql).toContain('ADD COLUMN `age` int NULL');
+    expect(sql).toContain("COMMENT '年龄'");
+    expect(sql).toContain('AFTER `id`');
+  });
+
+  it('builds kingbase alter preview without mysql-only syntax', () => {
+    const sql = buildAlterTablePreviewSql(buildInput({
+      dbType: 'kingbase',
+      tableName: 'public.users',
+    }));
+
+    expect(sql).toContain('ALTER TABLE public.users');
+    expect(sql).toContain('ADD COLUMN age int');
+    expect(sql).toContain("COMMENT ON COLUMN public.users.age IS '年龄';");
+    expect(sql).not.toContain('`');
+    expect(sql).not.toContain('AFTER');
+    expect(sql).not.toContain(' FIRST');
+  });
+});