问一批："!“如果-否则MYSQL中的运算符

EN

  setlocal enableextensions 
  setlocal enableDelayedExpansion

  if !a! EQU !b! (%sql_PATH%\mysql -uroot -e "DROP DATABASE '!a!';")

  @echo off
  setlocal enableextensions 
  setlocal enableDelayedExpansion

  set sql_PATH=C:\wamp\bin\mysql\mysql5.5.24\bin
  set sql_DIR=C:\Users\neca\Desktop\db_test\databases\db_name
  set "dbUser=root"
  set "dbPass="

  set LF=^
  %empty line%
  %empty line%

  cd %sql_DIR%
  FOR /D %%b IN (*) DO %sql_PATH%\mysql -u%dbUser% -e "CREATE DATABASE IF NOT EXISTS %%b;" && set "b=!b!%%b!LF!" 

  echo !b!

  cd %sql_PATH%
  for /f "usebackq delims=" %%a in (`
    %sql_PATH%\mysql -u%dbUser% -e "SHOW DATABASES LIKE '%%sample%%';" 
    ^| findstr /l /v /c:"Database" /c:"information_schema"
`) do (set "a=!a!%%a!LF!")

  echo !a!


  if !a! EQU !b! (%sql_PATH%\mysql -uroot -e "DROP DATABASE !a!;")

  pause