sql执行中无法解释的错误

Question

UPDATE amc_machine b
    SET     with_parts = a.with_parts,
        amc_validity_upto = a.amc_validity_upto
        FROM (SELECT CASE
            WHEN count(*) > 0 THEN (SELECT DISTINCT ON (machine_id) with_parts, amc_validity_upto, machine_id
                        FROM amc_amcdetail
                        WHERE machine_id = 2 AND id != 1
                        ORDER BY machine_id, amc_validity_upto DESC)
            WHEN count(*) = 0 THEN (SELECT FALSE AS with_parts, NULL AS amc_validity_upto, 2 AS machine_id)
            END AS a
            FROM (SELECT DISTINCT ON (machine_id) with_parts, amc_validity_upto, machine_id
                        FROM amc_amcdetail
                        WHERE machine_id = 2
                        ORDER BY machine_id, amc_validity_upto
                        ) AS T) AS foo
        WHERE a.machine_id = b.id

显示的错误是

ERROR:  subquery must return only one column
LINE 5:    WHEN count(*) > 0 THEN (SELECT DISTINCT ON (machine_id) w...

任何人都可以说出似乎是什么问题。 基本上，查询是使用表a中的数据更新表b（如果存在），否则使用null更新，false视情况而定。

查询在独立时执行。我正在使用Postgres 9.3，但部署将在postgres9.1

Answer 1

子查询返回3列

SELECT DISTINCT ON (machine_id) with_parts, amc_validity_upto, machine_id

让它只返回一个

SELECT DISTINCT ON (machine_id) with_parts