notes/db/sql/postgres/feature/dml/constraints.txt

create table ...
    column devault <val>|<func-call-with-params>

    column ... check ( val = 1 or val = 2)

    cosntraint constraint-name unique (col1, col2, ...)

    primary key (col1, col2, ...)

    foreign key (some-column) references another-table ( another-table-column-name )
        or
    some-column references another-table ( another-table-column-name )
        on update|delete cascade|restrict|no|[set default]

alter table table-name
    drop constraint constraint-name,
    alter column column-name set data type integer
        using (case when column-name = 'Economy' then 1
                    when column-name = 'Business' then 2
                    else 3
               end)
    ...
    rename constraint constraint-name to new-constraint-name
    ...
    add unique (column-name)

insert into
    values (...) 
    on conflict on constraint ..._tmp_key
        do update set field1 = excluded.field1,
                      field2 = excluded.field2
    "excluded" db table is maintained by db internally