copy data from one column to another postgres code example

Example 1: copy value from one column to another postgres

UPDATE table SET columnB = columnA;

Example 2: copy column from one table to another without column duplicate postgres

//Postgres Database Query
insert into table1(column1,column2)
select c1,c2
  from table2 a
 where not exists ( select 0 from table1 b where b.id = a.id )