如果表不存在

Question

我只想在表存在的情况下执行查询。

如果表不存在，请不要执行任何操作。

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。

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

所以，我想是这样的：

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) { 

我不知道有关使用DO NOTHING，而现在我收到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) { 

Ιt工作时，表是空的，但是当我填写表中应做到select * from table它抛出error: query has no destination for result data

Answer 1

海外商品会有用callback代替，水木清华这样的：

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); 
      } 
     }); 
     } 
});