Sybase Inc. Chairman Mitchell Kertzman resigned from the database software maker to become president and chief executive of Network Computer Inc., a software maker majority-owned by Oracle Corp. Kertzman, 49, replaces David Roux, Oracle executive vice president of corporate development, who becomes chairman of closely held NCI. Sybase President and Chief Executive John Chen assumed the chairman title as well at Sybase. NCI, jointly owned by Oracle and Netscape Communications Corp., makes software to operate machines without hard drives that receive their applications from a central computer. Network computers have been slow to take off amid the soaring popularity of sub-$1,000 personal computers. NCI, which competes against Microsoft Corp., has shifted focus to sell more software for digital set-top boxes, such as the new-generation devices being introduced by the cable television industry. Shares of Emeryville-based Sybase rose 75 cents to close at $6.75 on Nasdaq.