sql - Understanding union query - Stack Overflow
2021年3月10日 · UNION ALL - Includes duplicates. UNION - Excludes duplicates. A UNION operation is different from a JOIN: A UNION concatenates result sets from two queries. But a UNION does not create individual rows from columns gathered from two tables. A JOIN compares columns from two tables, to create result rows composed of columns from two tables.
sql - What is the difference between JOIN and UNION? - Stack …
2009年5月25日 · The SQL JOIN clause is used to combine records from two or more tables in a database. A JOIN is a means for combining fields from two tables by using values common to each. The SQL UNION operator combines the result of two or more SELECT statements. Each SELECT statement within the UNION must have the same number of columns.
sql - UNION with WHERE clause - Stack Overflow
2017年3月20日 · I'm doing a UNION of two queries on an Oracle database. Both of them have a WHERE clause. Is there a difference in the performance if I do the WHERE after UNIONing the queries compared to performing the UNION after WHERE clause? For example: SELECT colA, colB FROM tableA WHERE colA > 1 UNION SELECT colA, colB FROM tableB WHERE colA …
SQL UNIONs that use the WITH statement - Stack Overflow
2011年3月21日 · How to use union in sql? 1. How do I union multiple WITH clause in SQL Server 2008? 2. SQL UNION syntax. 1.
WHERE statement after a UNION in SQL? - Stack Overflow
2011年3月27日 · SELECT * FROM ( SELECT * FROM Table1 WHERE Field1 = Value1 UNION SELECT * FROM Table2 WHERE Field1 = Value2 ) AS t WHERE Field2 = Value3 Basically, the UNION is looking for two complete SELECT statements to combine, and the WHERE clause is part of the SELECT statement.
Sql Server union with If condition - Stack Overflow
2012年11月1日 · You can add your condition to the query like this. the section part of the union will simply return no results if your test condition is false: DECLARE @tmpValue SET @tmpValue = 0 -- it will be change SELECT * FROM Animal WHERE AniActive = 1 UNION SELECT * FROM Animal WHERE.Active = 0 and @tmpValue > 0
sql - What is the difference between UNION and UNION ALL
2008年9月8日 · UNION removes duplicates, whereas UNION ALL does not.. In order to remove duplicates the result set must be sorted, and this may have an impact on the performance of the UNION, depending on the volume of data being sorted, and the settings of various RDBMS parameters ( For Oracle PGA_AGGREGATE_TARGET with …
mysql - SELECT UNION as DISTINCT - Stack Overflow
2012年1月9日 · The reason for this is, SQL cannot guess which values of Value to leave for you—and which to drop. That's something you need to define by yourself. That's something you need to define by yourself. Try using GROUP BY to ensure ID is not repeated, and any aggregate (here MIN , as in your example it was the minimum that survived) to select a ...
sql - SELECT INTO USING UNION QUERY - Stack Overflow
UNION in SQL queries. 41. Select from union in SQL Server. 0. UNION select sql statement returns duplicate ...
sql - Union select statements horizontally - Stack Overflow
see SQL Fiddle with Demo. If you don't need them in the same row, then use UNION ALL: select id, aname, aid, 'Animal' tbl from animal union all select id, tname, tid, 'Transport' from transport see SQL Fiddle with Demo. Edit #1, here is a version with an UNPIVOT and PIVOT: