在Vim中如何切换命令和插入模式？(How do I switch between command and insert mode in Vim?)

UPDATE mytable
SET myfield = SUBSTRING(myfield, 1, LENGTH(myfield)-4) ;
 
 如果你想在截断之前检查字段是否以'.php'结尾，你可以添加这个条件： 
 
UPDATE mytable
SET myfield = SUBSTRING(myfield, 1, LENGTH(myfield)-4)
WHERE RIGHT(myfield, 4) = '.php' ;
 
 哦，还有LEFT()可以用来代替SUBSTRING() 
 
 而CHAR_LENGTH()应该用来代替LENGTH()因为它是多字节安全的（而LENGTH()不是）： 
 
UPDATE mytable
SET myfield = LEFT(myfield, CHAR_LENGTH(myfield)-4)
WHERE RIGHT(myfield, 4) = '.php' ;

UPDATE mytable
SET myfield = SUBSTRING(myfield, 1, LENGTH(myfield)-4) ;
 
If you want to also check whether the field ends with '.php' before truncating, you can add this condition:
 
UPDATE mytable
SET myfield = SUBSTRING(myfield, 1, LENGTH(myfield)-4)
WHERE RIGHT(myfield, 4) = '.php' ;
 
Oh, there's also LEFT() which can be used instead of SUBSTRING() as well
 
And CHAR_LENGTH() should be used instead of LENGTH() as it is multi-byte safe (while LENGTH() is not):
 
UPDATE mytable
SET myfield = LEFT(myfield, CHAR_LENGTH(myfield)-4)
WHERE RIGHT(myfield, 4) = '.php' ;