Q. When did Columbia become the capital of South Carolina?

On 22 March 1786 an act was ratified "for purpose of building a town and for moving the seat of government thereto."  On 1 December 1789 the records of the state were moved from Charleston to Columbia. On 4 January 1790 the legislature met for the first time at Columbia in the unfinished statehouse.

Sources: George C. Rogers, A South Carolina Chronology, 1497-1992; John Clements, ed., South Carolina Facts.