![]() ![]() The following picture illustrates the main difference between UNION and JOIN: SQL Server UNION examples In other words, join appends the result sets horizontally while union appends the result set vertically. The join such as INNER JOIN or LEFT JOIN combines columns from two tables while the UNION combines rows from two queries. In other words, the UNION operator removes the duplicate rows while the UNION ALL operator includes the duplicate rows in the final result set. However, if you want to retain the duplicate rows, you need to specify the ALL keyword is explicitly as shown below: query_1 UNION ALLīy default, the UNION operator removes all duplicate rows from the result sets. The following Venn diagram illustrates how the result set of the T1 table unions with the result set of the T2 table: UNION vs. The data types of the corresponding columns must be the same or compatible.The number and the order of the columns must be the same in both queries.The following are requirements for the queries in the syntax above: The following illustrates the syntax of the SQL Server UNION: query_1Ĭode language: SQL (Structured Query Language) ( sql ) SQL Server UNION is one of the set operations that allow you to combine results of two SELECT statements into a single result set which includes all the rows that belong to the SELECT statements in the union. ![]() ![]() Introduction to SQL Server UNION operator To allow success the number of columns, data types, and data order in each SELECT must be a match.Summary: in this tutorial, you will learn how to use the SQL Server UNION to combine the results of two or more queries into a single result set. UNION performs a deduplication step before returning the final results, UNION ALL retains all duplicates and returns the full, concatenated results. We have seen that UNION and UNION ALL are useful to concatenate data sets and to manage whether or not we retain duplicates. This will result in an error being returned. While we have the correct number of columns, they are now queried in the wrong order in the second SELECT statement and thus the data types also do not match up. ![]() The next 2 examples shows that we would return results whether we used UNION or UNION ALL since all required criteria are met. The columns retrieved must be of similar data types.The columns retrieved must be in the same order in each SELECT statement.There must be the same number of columns retrieved in each SELECT statement to be combined.UNION or UNION ALL have the same basic requirements of the data being combined: *Note: In both of these examples, the field names from the first SELECT statement are retained and used as the field names in the result set. If we were to now perform the UNION ALL on the same data set, the query would skip the deduplication step and return the results shown. UNION first performs a sorting operation and eliminates of the records that are duplicated across all columns before finally returning the combined data set. UNION ALL keeps all of the records from each of the original data sets, UNION removes any duplicate records. UNION ALL: keeps all records, including duplicates.The main difference between UNION and UNION ALL is that: This allows us to write multiple SELECT statements, retrieve the desired results, then combine them together into a final, unified set. UNION and UNION ALL are SQL operators used to concatenate 2 or more result sets. What is the difference between UNION and UNION ALL ![]()
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |