下面的选择查询将返回true/false，使用EXISTS()函数。

EXISTS(): EXISTS的参数是一个任意的SELECT语句，或者 子查询。计算子查询以确定它是否返回 任何行。如果它至少返回一行，则EXISTS的结果为 “真正的”;如果子查询不返回任何行，EXISTS的结果为 “假”

SELECT EXISTS(SELECT  column_name 
                FROM  information_schema.columns 
               WHERE  table_schema = 'public' 
                 AND  table_name = 'x' 
                 AND  column_name = 'y'); 

并使用下面的动态SQL语句修改表

DO
$$
BEGIN
IF NOT EXISTS (SELECT column_name 
                 FROM  information_schema.columns 
                WHERE  table_schema = 'public' 
                  AND  table_name = 'x' 
                  AND  column_name = 'y') THEN
ALTER TABLE x ADD COLUMN y int DEFAULT NULL;
ELSE
RAISE NOTICE 'Already exists';
END IF;
END
$$

只需检查查询是否返回了一个column_name。

如果不是，执行如下命令:

ALTER TABLE x ADD COLUMN y int;

你把一些有用的东西放在x和y上，当然还有一个合适的数据类型，我用的是int。

下面的函数将检查列是否存在，如果返回适当的消息，否则它将把列添加到表中。

create or replace function addcol(schemaname varchar, tablename varchar, colname varchar, coltype varchar)
returns varchar 
language 'plpgsql'
as 
$$
declare 
    col_name varchar ;
begin 
      execute 'select column_name from information_schema.columns  where  table_schema = ' ||
      quote_literal(schemaname)||' and table_name='|| quote_literal(tablename) || '   and    column_name= '|| quote_literal(colname)    
      into   col_name ;   

      raise info  ' the val : % ', col_name;
      if(col_name is null ) then 
          col_name := colname;
          execute 'alter table ' ||schemaname|| '.'|| tablename || ' add column '|| colname || '  ' || coltype; 
      else
           col_name := colname ||' Already exist';
      end if;
return col_name;
end;
$$

你可以这样做。

ALTER TABLE tableName drop column if exists columnName; 
ALTER TABLE tableName ADD COLUMN columnName character varying(8);

因此，如果该列已经存在，它将删除该列。然后将列添加到特定的表中。

可以添加到迁移脚本调用函数，并在完成时删除。

create or replace function patch_column() returns void as
$$
begin
    if exists (
        select * from information_schema.columns
            where table_name='my_table'
            and column_name='missing_col'
     )
    then
        raise notice 'missing_col already exists';
    else
        alter table my_table
            add column missing_col varchar;
    end if;
end;
$$ language plpgsql;

select patch_column();

drop function if exists patch_column();

Postgres 9.6新增ALTER TABLE tbl ADD COLUMN IF NOT EXISTS column_name。 所以这已经过时了。您可以在较旧的版本或变体中使用它来检查列名以外的其他内容。

CREATE OR REPLACE function f_add_col(_tbl regclass, _col  text, _type regtype)
  RETURNS bool
  LANGUAGE plpgsql AS
$func$
BEGIN
   IF EXISTS (SELECT FROM pg_attribute
              WHERE  attrelid = _tbl
              AND    attname  = _col
              AND    NOT attisdropped) THEN
      RETURN false;
   ELSE
      EXECUTE format('ALTER TABLE %s ADD COLUMN %I %s', _tbl, _col, _type);
      RETURN true;
   END IF;
END
$func$;

电话:

SELECT f_add_col('public.kat', 'pfad1', 'int');

成功返回true，否则返回false(列已经存在)。 对于无效的表名或类型名引发异常。

为什么是另一个版本?

This could be done with a DO statement, but DO statements cannot return anything. And if it's for repeated use, I would create a function. I use the object identifier types regclass and regtype for _tbl and _type which a) prevents SQL injection and b) checks validity of both immediately (cheapest possible way). The column name _col has still to be sanitized for EXECUTE with quote_ident(). See: Table name as a PostgreSQL function parameter format() requires Postgres 9.1+. For older versions concatenate manually: EXECUTE 'ALTER TABLE ' || _tbl || ' ADD COLUMN ' || quote_ident(_col) || ' ' || _type; You can schema-qualify your table name, but you don't have to. You can double-quote the identifiers in the function call to preserve camel-case and reserved words (but you shouldn't use any of this anyway). I query pg_catalog instead of the information_schema. Detailed explanation: How to check if a table exists in a given schema Blocks containing an EXCEPTION clause are substantially slower. This is simpler and faster. The manual:

提示 包含EXCEPTION子句的块要多得多 进出的成本比一个没有的街区要高。 因此，如果没有必要，不要使用EXCEPTION。