clear 是清空已有数据；
replace是替代已有数据。
clear 一般接在use后面，但replace基本接在save后面。

感谢感谢

看帮助解释
use 后面也没有replace参数啊


Title

    [D] use -- Load Stata dataset
Syntax
    Load Stata-format dataset
        use filename [, clear nolabel]

    Load subset of Stata-format dataset
        use [varlist] [if] [in] using filename [, clear nolabel]

Menu
    File > Open...
Description
    use loads into memory a Stata-format dataset previously saved by save.  If filename is specified
    without an extension, .dta is assumed.  If your filename contains embedded spaces, remember to
    enclose it in double quotes.

    In the second syntax for use, a subset of the data may be read.
Options
    clear specifies that it is okay to replace the data in memory, even though the current data have not been saved to disk.

    nolabel prevents value labels in the saved data from being loaded.  It is unlikely that you will ever want to specify this option.

Examples
    . use http://www.stata-press.com/data/r13/auto
    . replace rep78 = 3 in 12

    . use http://www.stata-press.com/data/r13/auto, clear
    . keep make price mpg rep78 weight foreign
    . save myauto

    . use make rep78 foreign using myauto
    . describe

    . use if foreign == 0 using myauto
    . tab foreign, nolabel
    . use using myauto if foreign==1
    . tab foreign, nolabel

那GENERATE 和REPLACE 如何结合用啊

先drop再gen，或者直接用replace。