2018年2月11日15:34:23
做PHP開發時，要寫一個SQL語句。就是如果表中存在相應的記錄時，就執行更新，不存在就執行插入。
記得以前做SQL SERVER時，有merge into 語句，可以按查詢條件，選擇要執行的操作，是插件還是更新。但是mysql好像沒這個功能。它是按主鍵是是否存在進行辨別的。
所以習慣性地寫了如下語句：
INSERT INTO readbook (readid,readpage,readcount,bookid,userid,readtime) VALUES ( (SELECT readid From readbook where userid=1 and bookid=1 and readpage=5),5,1,1,1,now()) ON duplicate key UPDATE readcount=readcount + 1,readtime=now()
結果提示錯誤，如下：
靜態分析： 分析時發現11個錯誤。 Unrecognized keyword. (near "key" at position 185) Unexpected token. (near "=" at position 209) Unexpected token. (near "readcount" at position 210) Unexpected token. (near "+" at position 220) Unexpected token. (near "1" at position 222) Unexpected token. (near "," at position 223) Unexpected token. (near "readtime" at position 224) Unexpected token. (near "=" at position 232) Unrecognized keyword. (near "now" at position 233) Unexpected token. (near "(" at position 236) Unexpected token. (near ")" at position 237) SQL 查詢： 文檔 INSERT INTO readbook (readid,readpage,readcount,bookid,userid,readtime) VALUES ( (SELECT readid From readbook where userid=1 and bookid=1 and readpage=5) ,5,1,1,1,now()) ON duplicate key UPDATE readcount=readcount + 1,readtime=now() MySQL 返回： 文檔 #1093 - You can't specify target table 'readbook' for update in FROM clause
於是更改語句：
INSERT INTO readbook (readid,readpage,readcount,bookid,userid,readtime) VALUES ( (SELECT * from (SELECT readid From readbook where userid=1 and bookid=1 and readpage=5) as a ),5,1,1,1,now()) ON duplicate key UPDATE readcount=readcount + 1,readtime=now()
提示插入成功。
之所以能成功是因爲，1093錯誤爲：但修改一個表的時候子查詢不能是同一個表，解決辦法：把子查詢再套一層，變成原來表的孫子查詢就可以了
其實，這個過程中，由於對Mysql不熟悉，我還遇到了其它很多問題。
比如
(SELECT * from (SELECT readid From readbook where userid=1 and bookid=1 and readpage=5) as a
必須有 as a ，因爲必須給表起個別名，但我以前使用SQL SERVER時，沒這個要求。
還有就是，原來我還這麼寫過
INSERT INTO readbook (readid,readpage,readcount,bookid,userid,readtime) VALUES ( (SELECT CASE WHEN a.readid>0 THEN a.readid ELSE NULL END from(SELECT readid From readbook where userid=1 and bookid=1 and readpage=3) as a) ,3,1,1,1,now()) ON duplicate key UPDATE readcount=readcount + 1,readtime=now()
後來逐漸嘗試，發現這個CASE WEHN是沒有必要的，貌似MYSQL能自動把查不到結果的值，以默認值代替。