Question

我一直依赖于CREATE VIEW和别名来创建SQL查询，并且我相信它可能是一种更有效的方法。有人可以提供任何一般建议吗？这是我最新的憎恶：

SELECT associations2.object_id, associations2.term_id, associations2.cat_ID, associations2.term_taxonomy_id
FROM (SELECT objects_tags.object_id, objects_tags.term_id, wp_cb_tags2cats.cat_ID, categories.term_taxonomy_id
    FROM (SELECT wp_term_relationships.object_id, wp_term_taxonomy.term_id, wp_term_taxonomy.term_taxonomy_id
        FROM wp_term_relationships
        LEFT JOIN wp_term_taxonomy ON wp_term_relationships.term_taxonomy_id = wp_term_taxonomy.term_taxonomy_id
        ORDER BY object_id ASC, term_id ASC) 
        AS objects_tags
    LEFT JOIN wp_cb_tags2cats ON objects_tags.term_id = wp_cb_tags2cats.tag_ID
    LEFT JOIN (SELECT wp_term_relationships.object_id, wp_term_taxonomy.term_id as cat_ID, wp_term_taxonomy.term_taxonomy_id
        FROM wp_term_relationships
        LEFT JOIN wp_term_taxonomy ON wp_term_relationships.term_taxonomy_id = wp_term_taxonomy.term_taxonomy_id
        WHERE wp_term_taxonomy.taxonomy = 'category'
        GROUP BY object_id, cat_ID, term_taxonomy_id
        ORDER BY object_id, cat_ID, term_taxonomy_id) 
        AS categories on wp_cb_tags2cats.cat_ID = categories.term_id
    WHERE objects_tags.term_id = wp_cb_tags2cats.tag_ID
    GROUP BY object_id, term_id, cat_ID, term_taxonomy_id
    ORDER BY object_id ASC, term_id ASC, cat_ID ASC) 
    AS associations2
LEFT JOIN categories ON associations2.object_id = categories.object_id
WHERE associations2.cat_ID <> categories.cat_ID
GROUP BY object_id, term_id, cat_ID, term_taxonomy_id
ORDER BY object_id, term_id, cat_ID, term_taxonomy_id

Answer 1

在对不同类型的连接进行一些研究以及使用别名的更好方法之后，我的SQL代码更加精简和高效（运行时间不长）。答案是双重的：使用正确的JOIN方法来减少返回的行数 - 在大多数情况下INNER JOIN特别有用（参见http://www.w3schools.com/sql/sql_join_inner.asp） - 并使用别名在多个列上连接同一个表。 / p>

后一种解决方案特别有用，因为我之前所做的主要是创建四个或五个视图（使用CREATE VIEW）以返回数据，然后根据其他条件过滤该数据。例如，我使用“CREATE VIEW view1 AS SELECT”语句返回行a，b，c和d，然后使用“SELECT a FROM view1 WHERE criteria”语句来过滤数据。有时这会重复多次。

完成相同任务的更有效方法是使用别名在同一个表上使用多个JOIN。结合大多数查询的INNER JOIN，它大大缩短了查询时间，并返回与更复杂的语句相同的行。

示例：

SELECT * 
FROM table1
INNER JOIN table2 AS alias1
  ON table1.col1 = alias1.col1
INNER JOIN table2 AS alias2
  ON table1.col2 = alias2.col2
WHERE col2 = 'some_condition'

这种方法可以节省生命。

如何基于CREATE VIEW减少长SQL查询？

1 个答案: