mysql - 选择是否存在否则插入？

Question

IF EXISTS (select * from users where username = 'something') THEN
    select id from users where username = 'something';
ELSE 
    insert into users (username) values ('something');
END IF;

Answer 1

你的陈述很好。唯一的问题是，您不能像普通查询一样使用它。IF类似或的控制结构WHILE只允许在存储过程或函数中使用。

只需创建一个这样的过程：

delimiter $$

create procedure select_or_insert()
begin

IF EXISTS (select * from users where username = 'something') THEN
    select id from users where username = 'something';
ELSE 
    insert into users (username) values ('something');
END IF;

end $$

并这样称呼它：

call select_or_insert();

而已。

Answer 2

你可以这样尝试：-

if exists( select * from mytable where field = 'something') 
begin
   select somefield from mytable where field = 'something';
end
else
 begin
    insert into users (username) values ('something');
end

或者

if not exists( select * from mytable where field = 'something') 
begin
    insert into users (username) values ('something');       
end
else
 begin
    select somefield from mytable where field = 'something';
 end

尽管上述两个查询都是相同的。

或尝试使用 if：

IF EXISTS (select * from mytable where users = 'something')
    select field from mytable where users = 'something'
else
    into mytable (users) values ('something')

这是SQLFIDDLEDEMO

Answer 3

基于公认的答案，但只需要一个 SELECT 调用（为 MySQL 编写）：

delimiter $$

create procedure select_or_insert()
begin

SET @myId := (SELECT id FROM users WHERE username = 'something');

IF (ISNULL(@myId))
    THEN 
        INSERT INTO users (username) VALUES ('something');                 
END IF;

end $$

LAST_INSERT_ID()如果需要，可以使用以下函数有效地选择新插入的元素：

SELECT @myId := LAST_INSERT_ID();