如果表不存在，什么也不做

Question

我只想在表存在时执行查询。

如果表不存在，什么也不做。

因此，我想检查表是否存在，然后检查表中是否有一些数据，如果有，请选择它们。

我想要这个/client.query( "SELECT * FROM mytable", function(err,res) {

所以，我尝试了类似的方法：

client.query("do"+
                    " $$"+
                    "begin"+
                    " if (select count(*) from information_schema.columns" +
                    "     where table_schema = 'public' " +
                    "    and table_name = 'mytable' )"+
                    " then "+
                    "DO NOTHING;"+
                    "else "+
                    "SELECT * FROM mytable;" +
                    "end if;"+
                    "end;"+
                    "$$"+
                    ";", function(err, res)  {

我不确定DO NOTHING 的使用，现在我收到error: syntax error at or near "NOTHING"

如果我使用 NOT：

 client.query("do"+
                        " $$"+
                        "begin"+
                        " if NOT (select count(*) = 0  from information_schema.columns" +
                        "     where table_schema = 'public' " +
                        "    and table_name = 'mytable' )"+
                        " then "+
                        "SELECT * FROM mytable;" +
                        "end if;"+
                        "end;"+
                        "$$"+
                        ";", function(err, res)  {

当表格为空时它可以工作，但是当我填满表格时应该做select * from table它会抛出error: query has no destination for result data

Answer 1

考虑改用callback，像这样：

client.query("select count(*) c from information_schema.columns" +
  "     where table_schema = 'public' " +
  "    and table_name = 'mytable'", function(err, res)  {
      if (parseInt(res.rows[0].c) > 0 ) {
        client.query("select * from PUBLIC.mytable", function(err1, res1)  {
          if (res1.rows.length == 0 ) {
            console.log('table is empty');
            } else{
          console.log(res1.rows[0].some_field);
          }
        });
      }
});