What is union clause in SQL?
The SQL UNION clause/operator is used to combine the results of two or more SELECT statements without returning any duplicate rows. To use this UNION clause, each SELECT statement must have. The same number of columns selected. The same number of column expressions.
What’s the difference between join and union in SQL?
UNION in SQL is used to combine the result-set of two or more SELECT statements. The data combined using UNION statement is into results into new distinct rows. JOIN combines data from many tables based on a matched condition between them. … Number of columns selected from each table should be same.
How does SQL union work?
The Union operator combines the results of two or more queries into a distinct single result set that includes all the rows that belong to all queries in the Union. In this operation, it combines two more queries and removes the duplicates. For example, the table ‘A’ has 1,2, and 3 and the table ‘B’ has 3,4,5.
How many UNIONs can you have in SQL?
I tested it for 8,192 UNIONs with SQL Server 2008 Enterprise. It executed OK. In SQL Server 2000, the max is something like 254…
Which is faster union or union all?
Both UNION and UNION ALL operators combine rows from result sets into a single result set. The UNION operator removes eliminate duplicate rows, whereas the UNION ALL operator does not. Because the UNION ALL operator does not remove duplicate rows, it runs faster than the UNION operator.
How do you optimize a query?
It’s vital you optimize your queries for minimum impact on database performance.
- Define business requirements first. …
- SELECT fields instead of using SELECT * …
- Avoid SELECT DISTINCT. …
- Create joins with INNER JOIN (not WHERE) …
- Use WHERE instead of HAVING to define filters. …
- Use wildcards at the end of a phrase only.
What is difference between where and having clause in SQL?
A HAVING clause is like a WHERE clause, but applies only to groups as a whole (that is, to the rows in the result set representing groups), whereas the WHERE clause applies to individual rows. A query can contain both a WHERE clause and a HAVING clause. … The HAVING clause is then applied to the rows in the result set.
What is difference between trigger and view?
difference between Triggers and views? Trigger is a special function/procedure defined to execute automatically when any data manipulation statement occurs on a table/view. View is a virtual table. Can be created on multiple tables.
What is data constraint?
Constraints are the rules enforced on the data columns of a table. These are used to limit the type of data that can go into a table. This ensures the accuracy and reliability of the data in the database. Constraints could be either on a column level or a table level.