union

\n UNION操作用于合并两个或多个 SELECT 语句的结果集。\n\n使用示例：\n\n~~~\nDb::field('name')\n ->table('think_user_0')\n ->union('SELECT name FROM think_user_1')\n ->union('SELECT name FROM think_user_2')\n ->select();\n~~~\n闭包用法：\n\n~~~\nDb::field('name')\n ->table('think_user_0')\n ->union(function($query){\n \t$query->field('name')->table('think_user_1');\n })\n ->union(function($query){\n \t$query->field('name')->table('think_user_2');\n })\n ->select();\n~~~\n或者\n\n~~~\nDb::field('name')\n ->table('think_user_0')\n ->union(['SELECT name FROM think_user_1','SELECT name FROM think_user_2'])\n ->select();\n~~~\n支持UNION ALL 操作，例如：\n\n~~~\nDb::field('name')\n ->table('think_user_0')\n ->union('SELECT name FROM think_user_1',true)\n ->union('SELECT name FROM think_user_2',true)\n ->select();\n~~~\n或者\n\n~~~\nDb::field('name')\n ->table('think_user_0')\n ->union(['SELECT name FROM think_user_1','SELECT name FROM think_user_2'],true)\n ->select();\n~~~\n每个union方法相当于一个独立的SELECT语句。\n\n> 注意：UNION 内部的 SELECT 语句必须拥有相同数量的列。列也必须拥有相似的数据类型。同时，每条 SELECT 语句中的列的顺序必须相同。\n