sql - 在 SQLite 触发器中测试 COUNT()>X

Question

如何比较 SQLite 中触发器内的 COUNT 结果？

到目前为止，这是我想出的代码：

CREATE TRIGGER mytrigger
BEFORE INSERT ON mytable
BEGIN
    SELECT CASE WHEN 
        SELECT COUNT (*) FROM mytable >= 3
    THEN
        RAISE(FAIL, "Activated - mytrigger.")
    END;
END;

它无法编译：

Error: near "SELECT": syntax error

如果我替换SELECT COUNT (*) FROM mytable >= 3为1 == 1，它编译得很好，并且触发器总是执行。

Answer 1

您需要在整个 SELECT 语句周围添加括号SELECT COUNT (*) FROM mytable

CREATE TRIGGER mytrigger
BEFORE INSERT ON mytable
BEGIN
    SELECT CASE WHEN 
        (SELECT COUNT (*) FROM mytable) >= 3
    THEN
        RAISE(FAIL, "Activated - mytrigger.")
    END;
END;

Answer 2

当计数大于 100 时，尝试使用此代码从 table2 中删除最后 50 行

CREATE **TRIGGER IF NOT EXISTS** delete_trigger
BEFORE INSERT ON table1 
  WHEN (SELECT COUNT(*) FROM table1) > 100
    BEGIN 
      delete From table1 where id not in (select id from table1 order by id desc limit 50; 
END;

Answer 3

尝试将计数放在括号内：

CREATE TRIGGER mytrigger
BEFORE INSERT ON mytable
BEGIN
    SELECT CASE WHEN 
        SELECT (COUNT (*) FROM mytable) >= 3
    THEN
        RAISE(FAIL, "Activated - mytrigger.")
    END;
END;