可以看delph的帮助，里面有这个问题详细说明：
A character string, also called a string literal or string constant, consists of a quoted string, a control string, or a combination of quoted and control strings. Separators can occur only within quoted strings.
A quoted string is a sequence of up to 255 characters from the extended ASCII character set, written on one line and enclosed by apostrophes. A quoted string with nothing between the apostrophes is a null string. Two sequential apostrophes in a quoted string denote a single character, namely an apostrophe. For example,
‘BORLAND‘ { BORLAND } //字符
‘You‘‘ll see‘ { You‘ll see } //两个‘连接在一起代表字符中的一个‘号.
‘‘‘‘ { ‘ } 同上.
‘‘ { null string } //空值
‘ ‘ { a space } //一个空格
示例：
SQL语句: where usename like ‘%张三%‘
str1 :=‘where ‘ + FieldName + ‘ like ‘ + ‘‘‘%‘ + SearchBuffer + ‘%‘‘‘