The best replacement for sql server express is mysql.

The best alternatives to sql server express are rated as following:
★★★★★  mysql
★★★★★  access
★★★  sql server
★  sql server standard
★  localdb
★  access database
★  sql server 2005
★  msde
  ms access
  oracle express
  access 2007
  microsoft access

Your alternative for sql server express is not listed? Suggest!
Speaking about sql server express, one probably shouldn't forget about management studio.

Other search terms related to sql server express and their ratings are:
★★★★★ management studio
★★★★ reporting services
★ ssis
You have in mind a companion for sql server express and it is not yet here? Propose!



Google Trends

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sql server express
mysql
access