Postgres 15

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select version();

version

PostgreSQL 15.0 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-10), 64-bit

SELECT 1

CREATE TABLE mytable(
   ID INTEGER  NOT NULL
  ,a  VARCHAR(4) NOT NULL
  ,b  VARCHAR(4) NOT NULL
  ,c  VARCHAR(3) NOT NULL
);
INSERT INTO mytable(ID,a,b,c) VALUES (1,'a1','b1','c1');
INSERT INTO mytable(ID,a,b,c) VALUES (2,'a2','b2','c2');

CREATE TABLE

INSERT 0 1

INSERT 0 1

select ID, (select max(col1) from (values (a), (b), (c)) as x(col1)) as max from mytable;

id
max

1
c1

2
c2

SELECT 2