我們在開發數據庫相關的邏輯過程中， 經常檢查表中是否已經存在這樣的一條記錄， 如果存在則更新或者不做操作， 如果沒有存在記錄，則需要插入一條新的記錄。
這樣的邏輯固然可以通過兩條sql語句完成。
SELECT COUNT(*) FROM xxx WHERE ID=xxx;
if (x == 0)
INSERT INTO xxx VALUES;
else
UPDATE xxx SET ;
但是這樣操作在性能上有所損失， 代碼結構感覺有點丑陋。
其實MySQL提供了可以在一個SQL語句中完成上述邏輯的支持。
官方文檔如下：
MySQL provides many extentions to SQL which help performance in many common use scenarios. Among these are INSERT ... SELECT, INSERT ... ON DUPLICATE KEY UPDATE, and REPLACE.
I rarely hesitate to use the above since they are so convenient and provide real performance benefits in many situations. MySQL has other keywords which are more dangerous, however, and should be used sparingly. These includeINSERT DELAYED, which tells MySQL that it is not important to insert the data immediately (say, e.g., in a logging situation). The problem with this is that under high load situations the insert might be delayed indefinitely, causing the insert queue to baloon. You can also give MySQL index hints about which indices to use. MySQL gets it right most of the time and when it doesn't it is usually because of a bad scheme or poorly written query.
重要的就是上面提到的 :
INSERT ... SELECT
INSERT ... ON DUPLICATE KEY UPDATE
INSERT ... ON DUPLICATE REPLACE
比如想往表中插入一條數據，如果表中沒有該條數據才插入，如果已經存在該條數據就不插入。
首先，在創建表時，將不需要重復的字段設置為unique，然后在插入時，使用insert ignore語句。
例如：（數據庫用的是mysql5）
創建一張表用來存儲用戶：
create table user_info
(
uid mediumint(10) unsigned NOT NULL auto_increment primary key,
last_name char(20) not null,
first_name char(20) not null,
unique ( last_name, first_name)
);
alter table anser add UNIQUE (last_name,first_name)
插入數據：
insert ignore into user_info (last_name,first_name) values ('x','y');
這樣一來，如果表中已經存在last_name='x'且first_name='y'的數據，就不會插入，如果沒有就會插入一條新數據。
上面的是一種用法， 也可以用 INSERT .... SELECT 來實現。
http://blog.csdn.net/langeldep/article/details/6241155
請博主參考 Replace語句
http://dev.mysql.com/doc/refman/5.0/en/replace.html