The best replacement for mssql is mysql.

The best alternatives to mssql are rated as following:
★★★★★  mysql
★★★★★  oracle
★★★  postgresql
★★  sybase
★★  postgres
★  msde
★  sqlsrv
★  sql
★  mysql syntax
★  mysql performance
  mysql benchmark
  access

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Speaking about mssql, one probably shouldn't forget about php.

Other search terms related to mssql and their ratings are:
★★★★★ php
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mssql
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