带有条件条件的Postgresql LEFT JOIN

Question

我想在PostgreSQL中使用CASE条件，以确定要连接的另一个表的哪一列。 我在这里，并且我认为可以解释我正在尝试做的事情。 感谢您的想法和意见：

SELECT hybrid_location.*,
       concentration
FROM   hybrid_location
CASE   WHEN EXTRACT(month FROM hybrid_location.point_time) = 1 
            THEN LEFT JOIN (SELECT jan_conc FROM io_postcode_ratios) ON
            st_within(hybrid_location.the_geom, io_postcode_ratios.the_geom) = true
       WHEN EXTRACT(month FROM hybrid_location.point_time) = 2 
            THEN LEFT JOIN (SELECT feb_conc FROM io_postcode_ratios) ON
            st_within(hybrid_location.the_geom, io_postcode_ratios.the_geom) = true
       ELSE LEFT JOIN (SELECT march_conc FROM io_postcode_ratios) ON
            st_within(hybrid_location.the_geom, io_postcode_ratios.the_geom) = true
       END AS concentration;

Answer 1

这是一个非常不寻常的查询，我认为这是无效的。 即使条件联接有效，查询计划者也很难优化。 可以将其重写以连接到单个联合表：

SELECT hybrid_location.*,
       concentration
FROM   hybrid_location
LEFT JOIN (
  SELECT 1 AS month_num, jan_conc AS concentration, io_postcode_ratios.the_geom
  FROM io_postcode_ratios
  UNION ALL
  SELECT 2 AS month_num, feb_conc AS concentration, io_postcode_ratios.the_geom
  FROM io_postcode_ratios
  UNION ALL
  SELECT 3 AS month_num, march_conc AS concentration, io_postcode_ratios.the_geom
  FROM io_postcode_ratios
) AS io_postcode_ratios ON
    EXTRACT(month FROM hybrid_location.point_time) = io_postcode_ratios.month_num
    AND ST_Within(hybrid_location.the_geom, io_postcode_ratios.the_geom)

组织io_postcode_ratios表的一种更好的方法（如果可以的话）是将每月*_conc列标准化为一个带有date或month列的conc列。 它将有更多的行，但更易于查询。