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plurals

POSelect parent row only if it has no children
primarykey
Id
6752776
data
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AnswerCount
7
ClosedDate
CommentCount
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CreationDate
2011-07-19T19:14:28.127
FavoriteCount
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2011-07-20T07:29:55.743
LastEditDate
2011-07-19T19:34:11.533
LastEditorUserId
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PostTypeId
1
Score
9
ViewCount
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LastEditorDisplayName
text
Body
I have a MySQL database in which table A has a one-to-many relation to table B, and I would like to select all rows in table B that have no children in table A. I have tried using <pre><code>SELECT id FROM A WHERE NOT EXISTS (SELECT * FROM B WHERE B.id=A.id) </code></pre> and <pre><code>SELECT id FROM A LEFT JOIN B ON A.id=B.id WHERE B.id IS NULL </code></pre> Both of these seem slow. Is there a faster query to achieve the same thing? In case this is relevant, in my database table A has about 500,000 rows and table B has about 3 to 4 million rows. Edit: For the actual tables in my database, explain gives me: <pre><code>+----+--------------------+------------------+-------+---------------+---------------------------+---------+------+---------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+--------------------+------------------+-------+---------------+---------------------------+---------+------+---------+--------------------------+ | 1 | PRIMARY | frontend_form471 | index | NULL | frontend_form471_61a633e8 | 32 | NULL | 671927 | Using where; Using index | | 2 | DEPENDENT SUBQUERY | SchoolData | index | PRIMARY | PRIMARY | 49 | NULL | 3121110 | Using where; Using index | +----+--------------------+------------------+-------+---------------+---------------------------+---------+------+---------+--------------------------+ </code></pre> for <pre><code>select number from frontend_form471 where not exists (select * from SchoolData where SchoolData.`f471 Application Number`=frontend_form471.number) </code></pre> and <pre><code>+----+-------------+------------------+-------+---------------+---------------------------+---------+------+---------+------------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------------+-------+---------------+---------------------------+---------+------+---------+------------------------------------------------+ | 1 | SIMPLE | frontend_form471 | index | NULL | frontend_form471_61a633e8 | 32 | NULL | 671927 | Using index; Using temporary | | 1 | SIMPLE | SchoolData | index | PRIMARY | PRIMARY | 49 | NULL | 3121110 | Using where; Using index; Not exists; Distinct | +----+-------------+------------------+-------+---------------+---------------------------+---------+------+---------+------------------------------------------------+ </code></pre> for <pre><code>select distinct number from frontend_form471 left join SchoolData on frontend_form471.number=SchoolData.`f471 Application Number` where SchoolData.`f471 Application Number` is NULL </code></pre> where in my case frontend_form471 is table A and SchoolData is table B Edit2: In table B (SchoolData) in my database, id is the first part of a two part primary key, so it is indexed and there are still multiple entries in B with the same id.
Tags
<mysql><sql>
Title
Select parent row only if it has no children
singulars
PostAcceptedAnswerId
1. PO
 singulars
 PostTypePostTypeId
 PTAnswer
PostParentId
1. This table or related slice is empty.
PostTypePostTypeId
1. PTQuestion
UserLastEditorUserId
1. USmurgatroid99
UserOwnerUserId
1. USmurgatroid99
plurals
PostLinksPostIdRelatedPostId
1. This table or related slice is empty.
PostLinksRelatedPostIdPostId
1. PL
 singulars
 LinkTypeLinkTypeId
 LTLinked
PostsAcceptedAnswerId
1. This table or related slice is empty.
PostsParentIdCreationDate
1. PO
 singulars
 PostTypePostTypeId
 PTAnswer
2. PO
 singulars
 PostTypePostTypeId
 PTAnswer
3. PO
 singulars
 PostTypePostTypeId
 PTAnswer
VotesPostIdCreationDate
1. VO
 singulars
 PostPostId
 POSelect parent row only if it has no children
 UserUserId
 This table or related slice is empty.
 VoteTypeVoteTypeId
 VTUpMod
2. VO
 singulars
 PostPostId
 POSelect parent row only if it has no children
 UserUserId
 This table or related slice is empty.
 VoteTypeVoteTypeId
 VTUpMod
3. VO
 singulars
 PostPostId
 POSelect parent row only if it has no children
 UserUserId
 This table or related slice is empty.
 VoteTypeVoteTypeId
 VTUpMod
CommentsPostId

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Guidance

A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.