One of the requirement for installing vRA is to have a database server for IaaS components. This DB server can reside on same host where IaaS role will be configured or can be external to IaaS host.
To see list of supported database that can be used for IaaS DB, please see vRealize Automation Support Matrix document from VMware.
Below table shows list of database that is suported with vRA 6.x/7.x
Note: Express versions of MSSQL are not supported. If you don’t have MSSQL enterprise edition for lab setup and you want to use express edition in your lab then please see this article by Rob Bastiaansen.