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我正在使用子查询和JOINs 处理复杂的 MySQL 查询，这是我的查询：

SELECT
    id,
    ancient_source_name,
    ancient_source_name_alt,
    ancient_source_type,
    ancient_source_type_id,
    IFNULL(IF(ancient_source_material = 'Unknown', NULL, ancient_source_material), '-') AS ancient_source_material,
    ancient_source_year,
    dynasty,
    ancient_period,
    author,
    IFNULL(CONCAT_WS(', ',
        IF(provenance_country = 'Unknown' OR provenance_country = 'Not Applicable', NULL, provenance_country),
        IF(provenance_locality = 'Unknown' OR provenance_locality = 'Not Applicable', NULL, provenance_locality),
        IF(provenance_place = 'Unknown' OR provenance_place = 'Not Applicable', NULL, provenance_place)
    ), '-') AS provenance_loc,
    IFNULL(CONCAT_WS(', ',
        IF(current_country = 'Unknown' OR current_country = 'Not Applicable', NULL, current_country),
        IF(current_locality = 'Unknown' OR current_locality = 'Not Applicable', NULL, current_locality),
        IF(current_place = 'Unknown' OR current_place = 'Not Applicable', NULL, current_place)
    ), '-') AS current_loc
FROM (
    SELECT
        hgn_ancient_sources.id AS id,
        ancient_source_name,
        IFNULL(ancient_source_name_alt, '-') AS ancient_source_name_alt,
        IFNULL(ancient_source_year, '-') AS ancient_source_year,
        ancient_source_type,
        ancient_source_type_id,
        ancient_source_material,
        dynasty,
        ancient_period,
        provenance_countries.country AS provenance_country,
        provenance_localities.locality AS provenance_locality,
        provenance_places.place AS provenance_place,
        current_countries.country AS current_country,
        current_localities.locality AS current_locality,
        current_places.place AS current_place,
        IFNULL(author_name, '-') AS author
    FROM hgn_ancient_sources
    JOIN hgn_ancient_source_types ON hgn_ancient_sources.ancient_source_type_id = hgn_ancient_source_types.id
    JOIN hgn_ancient_source_materials ON hgn_ancient_sources.ancient_source_material_id = hgn_ancient_source_materials.id
    JOIN hgn_dynasties ON hgn_ancient_sources.ancient_source_dynasty_id = hgn_dynasties.id
    JOIN hgn_ancient_periods ON hgn_ancient_sources.ancient_source_period_id = hgn_ancient_periods.id
    LEFT JOIN junc_ancient_source_has_author ON hgn_ancient_sources.id = junc_ancient_source_has_author.ancient_source_id
    LEFT JOIN hgn_authors ON junc_ancient_source_has_author.author_id = hgn_authors.id
    JOIN junc_place_has_location AS provenance_place_has_location ON hgn_ancient_sources.ancient_source_provenance_place_id = provenance_place_has_location.id
    JOIN junc_place_has_location AS current_place_has_location ON hgn_ancient_sources.ancient_source_current_place_id = current_place_has_location.id
    JOIN junc_locality_has_country AS provenance_locality_has_country ON provenance_place_has_location.location_id = provenance_locality_has_country.id
    JOIN junc_locality_has_country AS current_locality_has_country ON current_place_has_location.location_id = current_locality_has_country.id
    JOIN hgn_places AS provenance_places ON provenance_place_has_location.place_id = provenance_places.id
    JOIN hgn_places AS current_places ON current_place_has_location.place_id = current_places.id
    JOIN hgn_localities AS provenance_localities ON provenance_locality_has_country.locality_id = provenance_localities.id
    JOIN hgn_localities AS current_localities ON current_locality_has_country.locality_id = current_localities.id
    JOIN hgn_countries AS provenance_countries ON provenance_locality_has_country.country_id = provenance_countries.id
    JOIN hgn_countries AS current_countries ON current_locality_has_country.country_id = current_countries.id
) AS subquery

除了这部分代码外，一切正常：

IFNULL(CONCAT_WS(', ',
    IF(provenance_country = 'Unknown' OR provenance_country = 'Not Applicable', NULL, provenance_country),
    IF(provenance_locality = 'Unknown' OR provenance_locality = 'Not Applicable', NULL, provenance_locality),
    IF(provenance_place = 'Unknown' OR provenance_place = 'Not Applicable', NULL, provenance_place)
), '-') AS provenance_loc,
IFNULL(CONCAT_WS(', ',
    IF(current_country = 'Unknown' OR current_country = 'Not Applicable', NULL, current_country),
    IF(current_locality = 'Unknown' OR current_locality = 'Not Applicable', NULL, current_locality),
    IF(current_place = 'Unknown' OR current_place = 'Not Applicable', NULL, current_place)
), '-') AS current_loc

当有 1/3 或 2/3 列具有UnknownorNot Applicable值时，我得到了很好的结果，但是当 3/3 列（例如current_country、current_locality、current_place）具有这两个值之一时，我得到空单元格，而不是-，应该预料到的。

我在代码中尝试了不同的更改，但没有任何效果。我应该提到，例如，查询的这一部分：

IFNULL(IF(ancient_source_material = 'Unknown', NULL, ancient_source_material), '-') AS ancient_source_material

工作完美，所以我的假设是功能存在一些问题CONCAT_WS()，但我找不到什么。

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1 回答 1

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CONCAT_WS()如果非空分隔符后的所有参数都是 ，则返回空字符串NULL。
见演示。

我没有在MySql 的文档中找到明确的说明（就像在SQL Server 的文档中明确说明的那样）。

您还可以使用NULLIF()检查结果是否为空字符串并将其更改为NULL在使用之前IFNULL()：

  IFNULL(NULLIF(CONCAT_WS(', ',
      IF(provenance_country = 'Unknown' OR provenance_country = 'Not Applicable', NULL, provenance_country),
      IF(provenance_locality = 'Unknown' OR provenance_locality = 'Not Applicable', NULL, provenance_locality),
      IF(provenance_place = 'Unknown' OR provenance_place = 'Not Applicable', NULL, provenance_place)
  ), ''), '-') AS provenance_loc

见演示。

于 2022-01-02T15:35:22.430 回答