请教一下像这样的情况SQL应该怎么写

Question

SELECT DISTINCT

T_SERIAL_SET_INFO.SET_IRON_NO,
T_PRODUCTS_M.PRODUCTS_HEAD,
T_SERIAL_SET_INFO.IRON_SET_DATE,
T_SERIAL_SET_INFO.SET_LIFT_COM

FROM T_SERIAL_SET_INFO

INNER JOIN T_SERIAL_INFO
ON T_SERIAL_SET_INFO.SET_IRON_NO=T_SERIAL_INFO.SERIAL_NO
INNER JOIN T_PRODUCTS_COMPONENT_M
ON T_SERIAL_INFO.PARTS_CODE=T_PRODUCTS_COMPONENT_M.PARTS_CODE

INNER JOIN T_PRODUCTS_M

ON T_PRODUCTS_COMPONENT_M.PRDT_CODE=T_PRODUCTS_M.PRDT_CODE

WHERE T_SERIAL_SET_INFO.IRON_NO='0123456789'
ORDER BY
CASE T_PRODUCTS_M.PRODUCTS_HEAD
WHEN 'P' THEN 20
WHEN 'A' THEN 30
WHEN 'AS' THEN 40
WHEN 'S' THEN 50
ELSE CAST(T_PRODUCTS_M.PRODUCTS_HEAD as integer)
END

错误信息是
ERROR: SELECT DISTINCTではORDER BYの式はSELECTリスト内になければなりません
LINE 15: CASE T_PRODUCTS_M.PRODUCTS_HEAD
^

（在用SELECT DISTINCT时，用ORDER BY排列的列必须在SELECT列表里）
我知道这个写法是错的，但是不知道该怎么改

Answer 1

SELECT DISTINCT
T_SERIAL_SET_INFO.SET_IRON_NO,
T_PRODUCTS_M.PRODUCTS_HEAD,
T_SERIAL_SET_INFO.IRON_SET_DATE,
T_SERIAL_SET_INFO.SET_LIFT_COM,CASE T_PRODUCTS_M.PRODUCTS_HEAD
WHEN 'P' THEN 20
WHEN 'A' THEN 30
WHEN 'AS' THEN 40
WHEN 'S' THEN 50
ELSE CAST(T_PRODUCTS_M.PRODUCTS_HEAD as integer)
FROM T_SERIAL_SET_INFO
INNER JOIN T_SERIAL_INFO
ON T_SERIAL_SET_INFO.SET_IRON_NO=T_SERIAL_INFO.SERIAL_NO
INNER JOIN T_PRODUCTS_COMPONENT_M
ON T_SERIAL_INFO.PARTS_CODE=T_PRODUCTS_COMPONENT_M.PARTS_CODE
INNER JOIN T_PRODUCTS_M
ON T_PRODUCTS_COMPONENT_M.PRDT_CODE=T_PRODUCTS_M.PRDT_CODE
WHERE T_SERIAL_SET_INFO.IRON_NO='0123456789'
ORDER BY
CASE T_PRODUCTS_M.PRODUCTS_HEAD
WHEN 'P' THEN 20
WHEN 'A' THEN 30
WHEN 'AS' THEN 40
WHEN 'S' THEN 50
ELSE CAST(T_PRODUCTS_M.PRODUCTS_HEAD as integer)
END

指定distinct的时候，order by子句中的项就必须出现在选择列表中 。