mysql-在空表上不存在插入选择

所以我有以下示例查询

INSERT INTO company (name)
SELECT 'test'
FROM company
WHERE
  NOT EXISTS (SELECT 'test' FROM company WHERE name = 'test');

但是,当表为空时,这将不起作用.

我以为这样的事情会起作用

INSERT INTO company (name)
SELECT 'test'
FROM company
WHERE
  (SELECT COUNT(*) FROM company) = 0
  OR
  NOT EXISTS (SELECT 'test' FROM company WHERE name = 'test');

但这也不起作用.有任何想法吗？

http://sqlfiddle.com/#!9/5747f0/1

解决方法:

仔细查看以下查询：

INSERT INTO company (name)
SELECT 'test'
FROM company
WHERE
    NOT EXISTS (SELECT 'test' FROM company WHERE name = 'test');

由于company表当前为空,因此无论WHERE EXISTS逻辑如何(完全正确),都不会从该查询返回任何记录.如果改用对偶表,您将看到查询正常工作：

INSERT INTO company (name)
SELECT 'test'
FROM dual
WHERE
     NOT EXISTS (SELECT 'test' FROM company WHERE name = 'test');