![]() ![]() īy default, UNION eliminates duplicate rows from the result set:. ![]() ![]() Each of the preceding two statements will return an integer first column, which results in type conversion of the dates to integers. Note: Before MySQL 4.1.1, the data type for each column of the UNION result is based only on the type of the column in the first SELECT, and values from corresponding column of the following SELECT statements are converted to that type. In both cases, the result is a string column. In the second statement, integers and dates are selected for the first column, strings and integers for the second column. In the first statement, strings and dates are selected for the second column. In each statement, the data type for each column of the result is determined from the selected values. (Normally, you write UNION such that corresponding columns do have the same types, but MySQL performs type conversion as necessary if they do not.) Columns are matched by position rather than by name, which is why the following two statements return different results, even though they select the same values from the two tables: The second and subsequent SELECT statements in the UNION must select the same number of columns, but corresponding columns need not have the same names or data types. The names for the columns of the UNION result come from the names of the columns in the first SELECT. For example, to select the integer column from each table, do this: mysql> SELECT i FROM t1 UNION SELECT i FROM t2 UNION SELECT i FROM t3 +-+| i |+-+| 1 || 2 || 3 || -1 || 100 || 200 |+-+ To write a UNION statement that combines multiple retrievals, just write several SELECT statements and put the keyword UNION between them. Tables t1 and t2 have integer and character columns, and t3 has date and integer columns. If you want to create a result set that combines the results from several queries, you can do so by using a UNION statement. For example, you can filter them differently using different WHERE clauses.Learn More Buy Performing Multiple-Table Retrievals with UNION Since you are writing two separate SELECT statements, you can treat them differently before appending. This is because most of the instances in which you'd want to use UNION involve stitching together different parts of the same dataset (as is the case here). While the column names don't necessarily have to be the same, you will find that they typically are. The columns must have the same data types in the same order as the first table.Both tables must have the same number of columns.In this particular case, there are no duplicate rows, so UNION ALL will produce the same results: SELECT * You'll likely use UNION ALL far more often than UNION. If you'd like to append all the values from the second table, use UNION ALL. More specifically, when you use UNION, the dataset is appended, and any rows in the appended table that are exactly identical to rows in the first table are dropped. Note that UNION only appends distinct values. The following query will display all results from the first portion of the query, then all results from the second portion in the same table: SELECT *įROM tutorial.crunchbase_investments_part1įROM tutorial.crunchbase_investments_part2 ![]() Let's try it out with the Crunchbase investment data, which has been split into two tables for the purposes of this lesson. Put differently, UNION allows you to write two separate SELECT statements, and to have the results of one statement display in the same table as the results from the other statement. SQL joins allow you to combine two datasets side-by-side, but UNION allows you to stack one dataset on top of the other. Starting here? This lesson is part of a full-length tutorial in using SQL for Data Analysis. ![]()
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