1. Not knowing in what order queries execute SQL has a low barrier to entry. You start writing queries — use a JOIN here and there, do some grouping and you’re already an expert (at least some people think so). But does the so-called expert even know in what order do SQL queries...
Scenario 2.a: Delete Duplicate rows but keep one using CTE We need to use the technique of Self Join initially to check for duplicate records containing different custid but same passport number. select distinct a.* from customers2 a join customers2 b on a.custid <> b.custid and a.CustN...
The rowid pseudocolumn is used to identify rows in the subquery because this is guaranteed to be unique. The WHERE clause of the outer query uses a > ANY condition to check for duplicates. It will delete any row that has a rowid greater than at least one other row. This ensures that a...
可以看到和关闭semijoin一样,对于delete优化器也是选择了exists,我们表是千万级别,全表扫描加锁,其它操作语句自然都会超时获取不到锁而失败。 semijoin semijoin翻译过来是半连接,是mysql针对in/exists子查询进行优化的一种技术,参见文档。 可以使用SHOW VARIABLES LIKE 'optimizer_switch';查看semijoin是否开启。 上面使...
DUPLICATED_MAP_KEY DUPLICATE_KEY 23512 The check constraint cannot be added, because the table contains rows that do not satisfy the constraint definition. DELTA_NEW_CHECK_CONSTRAINT_VIOLATION DELTA_NEW_NOT_NULL_VIOLATION 23K01 MERGE cardinality violation MERGE_CARDINALITY_VIOLATIONClass 25: invalid tra...
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3.1 inner join时产生duplicates Table1 Table2 SELECTDISTINCT*FROMTable1#dist(t1)和t2的所有都选(仅限于t1有完全dup的rows),但是会出现duplicatesINNERJOINTable2ONTable1.id=Table2.id; Result 3.2 使用group by: SELECT*FROMTable1INNERJOINTable2ONTable1.id=Table2.idGROUPBYTable1.id,Table1.name;#这俩...
HTBUILD_JOIN 在哈希聯結的輸入端同步處理哈希表的建置時,會發生平行批次模式計劃。 如果等候過多且無法藉由調整查詢來減少(例如新增索引),請考慮調整平行處理原則的成本閾值,或降低平行處理原則的程度。 適用於:Azure SQL Database、具有 always-up-to-date 更...
13.JOIN：用于将多个表连接起来进行查询。14.LEFT JOIN、RIGHT JOIN、INNER JOIN：分别用于进行左连接、...
Delete top 1000 rows from Table in 2 batches Delete top n rows using join delete with except Deleted Microsoft SQL Server folder from C: Drive ... Deleting a database in RESTRICTED MODE Deleting all tables in a schema Deleting all tables in database older than 14 days Deleting duplicate re...