我怎样才能合并联接？答案

【问题标题】：How can i merge joins?我怎样才能合并联接？
【发布时间】：2017-11-17 13:08:48
【问题描述】：

我有以下查询，但我想改进（性能方面）：

.sentence_id 和 .project_id 是索引。

我现在的目标是尝试将同一张表上的 2 个连接合并为一个（如果可能）我该怎么做？

SELECT rs.project_id,
               ms.id AS raw_sentence_id,
               rs.sentence AS raw_sentence_text,
               rs.sentence_id AS raw_sentence_text_id,
               rst.tokens AS tokenization_information,
               ns.sentence_id AS normalized_sentence_id,
               ns.sentence AS normalized_sentence_text,
               nst.token_indices
          FROM manual_sentences ms
          JOIN sentences rs ON (ms.original_sentence_id = rs.sentence_id AND rs.project_id = 623483)
     LEFT JOIN sentence_tokens rst ON (ms.original_sentence_id = rst.sentence_id AND rst.project_id = 623483)
          JOIN sentences ns ON (ms.sentence_id = ns.sentence_id AND ns.project_id = 623483)
     LEFT JOIN sentence_tokens nst ON (ms.sentence_id = nst.sentence_id AND nst.project_id = 623483)
     WHERE ms.project_id = 623483

【问题讨论】：

标签： mysql join left-join

【解决方案1】：

你应该可以使用连词来做到这一点：

SELECT rs.project_id,
               ms.id AS raw_sentence_id,
               rs.sentence AS raw_sentence_text,
               rs.sentence_id AS raw_sentence_text_id,
               rst.tokens AS tokenization_information,
               ns.sentence_id AS normalized_sentence_id,
               ns.sentence AS normalized_sentence_text,
               nst.token_indices
          FROM manual_sentences ms
          JOIN sentences rs ON (ms.original_sentence_id = rs.sentence_id AND rs.project_id = 623483)
     LEFT JOIN sentence_tokens rst ON (ms.original_sentence_id = rst.sentence_id AND ms.sentence_id = rst.sentence_id AND rst.project_id = 623483)
          JOIN sentences ns ON (ms.sentence_id = ns.sentence_id AND ns.project_id = 623483)
     WHERE ms.project_id = 623483

【讨论】：

ms.original_sentence_id 和 ms.sentence_id 可以不同。当他们在时，我需要来自 2 个单独行的信息
那么在这种情况下，我认为您将需要两个联接。