4.update set 語句是mysql最常用的修改和更新語句，它更新信息時也會覆蓋（刪除）舊的信息。
A? update set 與where搭配使用，變更某些記錄：
update +表名 +set+ 變更后的信息 +where子句；
例如：
update?stu?set?birth=1988,department='中文系'?where?id=9????and?name='張三';
注意：如果變更信息后面沒有加where子句指定其變更的內(nèi)容，那么update set語句就會把這個字段中的所有信息全部更新，修改。
例如
mysql>?select?*?from?c1score; +-------+------+ |?score?|?s????| +-------+------+ |????56?|????1?| |????79?|????2?| |????91?|????3?| |????46?|????5?| |????35?|????6?| +-------+------+ 5?rows?in?set?(0.08?sec) mysql>?update?c1score?set?score=score+8; Query?OK,?5?rows?affected?(0.13?sec) Rows?matched:?5??Changed:?5??Warnings:?0 mysql>?select?*?from?c1score; +-------+------+ |?score?|?s????| +-------+------+ |????64?|????1?| |????87?|????2?| |????99?|????3?| |????54?|????5?| |????43?|????6?| +-------+------+???????????????????????????????????????????????????????????????????????????????????????????????????? 5?rows?in?set?(0.00?sec)
? ?
5用alter語句來刪除字段：
A?? alter table 表名 drop 字段名；
mysql>?alter?table?c1score?drop?s; Query?OK,?0?rows?affected?(1.80?sec) Records:?0??Duplicates:?0??Warnings:?0 mysql>?select?*?from?c1score; +-------+ |?score?| +-------+ |????64?| |????87?| |????99?| |????54?| |????43?| +-------+ 5?rows?in?set?(0.00?sec)
B?? 用alter來刪除索引：
mysql>?drop?index?idx_4a?on?4a;???? Query?OK,?0?rows?affected?(0.41?sec)???? Records:?0??Duplicates:?0??Warnings:?0
C? 用alter來刪除主鍵：
mysql>?alter?table?sc3?drop?primary?key; Query?OK,?17?rows?affected?(1.00?sec) Records:?17??Duplicates:?0??Warnings:?0
D 用alter來刪除，更新表名：
mysql>?alter?table?sc3?rename?to?gyssc; Query?OK,?0?rows?affected?(0.30?sec) mysql>?select?*?from?sc3; ERROR?1146?(42S02):?Table?'trains.sc3'?doesn't?exist
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