Question

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，这不起作用。

我认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');

但它也没有用。有什么想法吗？

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

Answer 1

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，将不会从此查询返回，无论WHERE EXISTS逻辑是什么，顺便说一句，这是完全正确的。如果您改为使用dual表，您将看到查询正常运行：

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

Answer 2

在prevent autoincrement on MYSQL duplicate insert

之前出现这种情况并不令人惊讶

我喜欢这个解决方案

INSERT INTO COMPANY (NAME) SELECT 'TEST' AS INNAME FROM DUAL
WHERE NOT EXISTS(
    SELECT NAME FROM COMPANY
    WHERE NAME = 'TEST'
    LIMIT 1
);

MariaDB [sandbox]> DROP TABLE IF EXISTS COMPANY;
Query OK, 0 rows affected (0.12 sec)

MariaDB [sandbox]> CREATE TABLE COMPANY (ID INT AUTO_INCREMENT PRIMARY KEY, NAME VARCHAR(10), UNIQUE (NAME));
Query OK, 0 rows affected (0.24 sec)

MariaDB [sandbox]>
MariaDB [sandbox]> INSERT INTO COMPANY (NAME) VALUES ('TEST');
Query OK, 1 row affected (0.08 sec)

MariaDB [sandbox]>
MariaDB [sandbox]> INSERT INTO COMPANY (NAME) SELECT 'TEST' AS INNAME FROM DUAL
    -> WHERE NOT EXISTS(
    ->     SELECT NAME FROM COMPANY
    ->     WHERE NAME = 'TEST'
    ->     LIMIT 1
    -> );
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

MariaDB [sandbox]>
MariaDB [sandbox]> INSERT INTO COMPANY (NAME) SELECT 'ABC' AS INNAME FROM DUAL
    -> WHERE NOT EXISTS(
    ->     SELECT NAME FROM COMPANY
    ->     WHERE NAME = 'ABC'
    ->     LIMIT 1
    -> );
Query OK, 1 row affected (0.10 sec)
Records: 1  Duplicates: 0  Warnings: 0

MariaDB [sandbox]>
MariaDB [sandbox]> SELECT * FROM COMPANY;
+----+------+
| ID | NAME |
+----+------+
|  2 | ABC  |
|  1 | TEST |
+----+------+
2 rows in set (0.00 sec)

Answer 3

看起来你在这里重新发明轮子。一种更简单的方法是将name定义为唯一键（或主键），然后使用insert ignore语法：

INSERT IGNORE INTO company(name) VALUES ('test')

mysql - 在空表上INSERT SELECT WHERE NOT EXISTS

3 个答案: