notes/db/sql/postgres/feature/lang/conditionals.txt

2022
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    https://proglib.tech/p/kak-namerenno-rasstavlennye-oshibki-pomogayut-sdelat-sql-kod-legko-podderzhivaemym-2022-11-10
        db=# INSERT INTO payment (method, amount) VALUES
            ('cash', 10000),
            ('credit_card', 12000),
            ('credit_card', 5000);
        INSERT 0 3

        db=# SELECT * FROM payment;
         id │   method    │ amount
        ────┼─────────────┼────────
          1 │ cash        │  10000
          2 │ credit_card │  12000
          3 │ credit_card │   5000
        (3 rows)

        -- calculate_commission.sql
        SELECT
            COUNT(*) AS payments,
            SUM(
                CASE method
                    WHEN 'cash' THEN 100
                    WHEN 'credit_card' THEN 30 + amount * 0.02
                END
            ) AS commission
        FROM
            payment;