Here’s what happens if we try to combine the TeacherName column with the StudentId column: SELECT TeacherName FROM Teachers However, if they don’t, the types and lengths of the columns in the UNION result take into account the values retrieved by all the SELECT statements. Selected columns listed in corresponding positions of each SELECT statement should have the same data type. Result: ERROR 1222 (21000): The used SELECT statements have a different number of columns Data Types SELECT StudentId, StudentName FROM Students Therefore, we can’t do the following: SELECT TeacherName FROM Teachers The number of columns returned by each SELECT statement must be the same. Result: ERROR 1054 (42S22): Unknown column 'TeacherName' in 'order clause' Number of Columns Here’s what happens if we don’t use the alias: (SELECT TeacherName t FROM Teachers) Note that if a column to be sorted uses an alias, then that column must be referenced by its alias (not the column name).Įxample: (SELECT TeacherName t FROM Teachers) Mind you, omitting the parentheses produces the same result as the one with parentheses: SELECT * FROM Teachers (SELECT * FROM Students ORDER BY StudentName ASC LIMIT 2)Įven when not using the ORDER BY clause within each SELECT statement, each SELECT statement should still be in parentheses, and the ORDER BY clause (or any LIMIT clause) should be after the last one. In this example, we take the previous example and order the combined results: (SELECT * FROM Teachers ORDER BY TeacherName ASC LIMIT 2) We can also use an ORDER BY clause on the whole query, so that the whole output is ordered together. Therefore, using ORDER BY without the LIMIT clause has no effect on the output. It only orders the results for the purpose of determining the subset of the selected rows to retrieve when applying the LIMIT clause. Note that when we do this, it doesn’t actually order the results for the output. (SELECT * FROM Students ORDER BY StudentName ASC LIMIT 2) When we use the ORDER BY clause in the individual SELECT statements within a UNION query, we need to enclose each SELECT statement inside parentheses: (SELECT * FROM Teachers ORDER BY TeacherName ASC LIMIT 2) We can use the ORDER BY clause in each SELECT statement and/or on the combined UNION query. Using the ORDER BY Clause in Union Queries That’s because the TeacherId columns contain different values, hence the rows are not duplicates. For example, two teachers called Bill are returned here whereas only one was returned in the earlier example. That’s because we’re selecting all columns in the table, which results in non-duplicates where there was previously a duplicate. You’ll notice that these statements return more rows than in our first example earlier. That’s the equivalent of the following query: SELECT * FROM Teachers TABLE Statementsįrom MySQL 8.0.19 we can use the UNION clause with the TABLE statement. We can see that both Cathys were returned and all three Bills were returned. This time we got twelve rows instead of the eight that we got in our first example. ![]() We can use the ALL keyword to include duplicate values in the results: SELECT TeacherName FROM Teachers So we get the same result that we got without the DISTINCT clause. Here’s an example that explicitly uses the DISTINCT clause: SELECT TeacherName FROM Teachers ![]() This is despite the fact that the combined tables actually contain two Warrens, two Cathys, and three Bills (there are two teachers called Cathy, a teacher and a customer called Warren, and two called Bill, as well as one student called Bill). So the above results contain just one each of Warren, Cathy and Bill. In other words, it returns only distinct values by default. ![]() The column names are taken from the first SELECT statement.īy default, the UNION clause implicitly applies a DISTINCT operation. We can insert the UNION clause between those two SELECT statements to return all teachers and students: SELECT TeacherName FROM Teachers Suppose we have the following tables: SELECT * FROM Teachers In MySQL, the UNION clause combines the results from multiple queries into a single result set.
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