PostgreSQL IF语句

Question

我如何在Postgres中进行此类查询？

IF (select count(*) from orders) > 0
THEN
  DELETE from orders
ELSE 
  INSERT INTO orders values (1,2,3);

Answer 1

尝试:

DO
$do$
BEGIN
IF EXISTS (SELECT FROM orders) THEN
   DELETE FROM orders;
ELSE 
   INSERT INTO orders VALUES (1,2,3);
END IF;
END
$do$

主要观点

• 标准SQL中没有过程元素.该IF语句是默认过程语言PL/pgSQL的一部分.您需要使用该DO命令创建函数或执行ad-hoc语句.

• ;在plpgsql中的每个语句的末尾都需要一个(最后的除外END).

• 你需要END IF;在IF声明的最后.

• 子选择需要用括号括起来:

  IF (SELECT count(*) FROM orders) > 0 ...
  

  要么:

  IF (SELECT count(*) > 0 FROM orders) ...
  

  但这相当且快得多:

  IF EXISTS (SELECT FROM orders) ...
  

替代

SELECT在这里你根本不需要额外的.这样做也快一点:

DO
$do$
BEGIN
DELETE FROM orders;
IF NOT FOUND THEN
   INSERT INTO orders VALUES (1,2,3);
END IF;
END
$do$

Answer 2

只是为了帮助,如果有人偶然发现像我这样的问题,如果你想在PostgreSQL中使用if,你使用"CASE"

select 
    case
        when stage = 1 then 'running'
        when stage = 2 then 'done'
        when stage = 3 then 'stopped'
    else 
        'not running'
    end as run_status from processes

Answer 3

来自文档

IF boolean-expression THEN
    statements
ELSE
    statements
END IF;

因此,在上面的示例中,代码应如下所示:

IF select count(*) from orders > 0
THEN
  DELETE from orders
ELSE 
  INSERT INTO orders values (1,2,3);
END IF;

你错过了:END IF;

Answer 4

您还可以将PL / pgSQL CASE的基本结构与匿名代码块过程块一起使用：

DO $$ BEGIN
    CASE
        WHEN boolean-expression THEN
          statements;
        WHEN boolean-expression THEN
          statements;
        ...
        ELSE
          statements;
    END CASE;
END $$;

参考文献：

1. http://www.postgresql.org/docs/current/static/sql-do.html
2. https://www.postgresql.org/docs/current/static/plpgsql-control-structures.html