SQLite 实现if not exist 类似功能的操作，具有很好的参考价值，希望对大家有所帮助。一起跟随小编过来看看吧。
需要实现：
if not exists(select * from ErrorConfig where Type='RetryWaitSeconds')
begin
insert into ErrorConfig(Type,Value1)
values('RetryWaitSeconds','3')
end
只能用：
insert into ErrorConfig(Type,Value1)
select 'RetryWaitSeconds','3'
where not exists(select * from ErrorConfig where Type='RetryWaitSeconds')
因为 SQLite 中不支持SP
补充：sqlite3中NOT IN 不好用的问题
在用sqlite3熟悉SQL的时候遇到了一个百思不得其解的问题，也没有在google上找到答案。虽然最后用“迂回”的方式碰巧解决了这个问题，但暂时不清楚原理是什么，目前精力有限，所以暂时记录下来，有待继续研究。
数据库是这样的：
CREATE TABLE book (
id integer primary key,
title text,
unique(title)
);
CREATE TABLE checkout_item (
member_id integer,
book_id integer,
movie_id integer,
unique(member_id, book_id, movie_id) on conflict replace,
unique(book_id),
unique(movie_id)
);
CREATE TABLE member (
id integer primary key,
name text,
unique(name)
);
CREATE TABLE movie (
id integer primary key,
title text,
unique(title)
);
该数据库包含了4个表:book, movie, member, checkout_item。其中，checkout_item用于保存member对book和movie的借阅记录，属于关系表。
问一：哪些member还没有借阅记录?
SQL语句(SQL1)如下：
1SELECT * FROM member WHERE id NOT IN(SELECT member_id FROM checkout_item);
得到了想要的结果。
问二：哪些book没有被借出?
这看起来与上一个是类似的，于是我理所当然地运行了如下的SQL语句(SQL2)：
1SELECT * FROM book WHERE id NOT IN(SELECT book_id FROM checkout_item);
可是——运行结果没有找到任何记录! 我看不出SQL2与SQL1这两条语句有什么差别，难道是book表的问题?于是把NOT去掉，运行了如下查询语句：
1SELECT * FROM book WHERE id IN(SELECT book_id FROM checkout_item);
正确返回了被借出的book，其数量小于book表里的总行数，也就是说确实是有book没有借出的。
接着google(此处省略没有营养的字)，没找到解决方案。可是，为什么member可以，book就不可以呢?它们之前有什么不同?仔细观察，发现checkout_item里的book_id和movie_id都加了一个unique，而member_id则没有。也许是这个原因?不用id了，换title试试：
SELECT * FROM book WHERE
title NOT IN(
SELECT title FROM book WHERE id IN(
SELECT book_id FROM checkout_item));
确实很迂回，但至少work了。。。
问题原因：当NOT碰上NULL
事实是，我自己的解决方案只不过是碰巧work，这个问题产生跟unique没有关系。邱俊涛的解释是，“SELECT book_id FROM checkout_item”的结果中含有null值，导致NOT也返回null。当一个member只借了movie而没有借book时，产生的checkout_item中book_id就是空的。
解决方案是，在选择checkout_item里的book_id时，把值为null的book_id去掉：
1SELECT * FROM book WHERE id NOT IN(SELECT book_id FROM checkout_item WHERE book_id IS NOT NULL);
总结
我在解决这个问题的时候方向是不对的，应该像调试程序一样，去检查中间结果。比如，运行如下语句，结果会包含空行：
1SELECT book_id FROM checkout_item
而运行下列语句，结果不会包含空行：
1SELECT member_id FROM checkout_item
这才是SQL1与SQL2两条语句执行过程中的差别。根据这个差别去google，更容易找到答案。当然了，没有NULL概念也是我“百思不得其解”的原因。
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