We’ll use the customers and employees tables in the sample database for the demonstration: The following picture illustrates the difference between UNION and JOIN: MySQL UNION and column alias examples JOINĪ JOIN combines result sets horizontally, a UNION appends result set vertically. UNION ALL SELECT id FROM t2 Code language: SQL (Structured Query Language) ( sql ) + -+Ħ rows in set ( 0.00 sec) Code language: SQL (Structured Query Language) ( sql )Īs you can see, the duplicates appear in the combined result set because of the UNION ALL operation. Because UNION ALL does not need to handle duplicates, it performs faster than UNION DISTINCT . If you use the UNION ALL explicitly, the duplicate rows, if available, remain in the result. The following Venn diagram illustrates the union of two result sets that come from t1 and t2 tables: ![]() The final result set contains the distinct values from separate result sets returned by the queries: + -+Ĥ rows in set ( 0.00 sec) Code language: SQL (Structured Query Language) ( sql )īecause the rows with value 2 and 3 are duplicates, the UNION removed them and kept only unique values. UNION SELECT id FROM t2 Code language: SQL (Structured Query Language) ( sql ) The following statement combines result sets returned from t1 and t2 tables: SELECT id FROM t1 INSERT INTO t2 VALUES ( 2),( 3),( 4) Code language: SQL (Structured Query Language) ( sql ) Let’s see the following sample tables: t1 and t2: DROP TABLE IF EXISTS t1 Second, the data types of columns must be the same or compatible.īy default, the UNION operator removes duplicate rows even if you don’t specify the DISTINCT operator explicitly.First, the number and the orders of columns that appear in all SELECT statements must be the same.To combine result set of two or more queries using the UNION operator, these are the basic rules that you must follow: Code language: SQL (Structured Query Language) ( sql ) ![]() The following illustrates the syntax of the UNION operator: SELECT column_list MySQL UNION operator allows you to combine two or more result sets of queries into a single result set. So my questions are, doesn't UNION and LIMIT play well together? And does anyone have a clever workaround, or an alternative solution/setup for this? If anyone is willing to help out, i can provide you with more details if needed.Summary : in this tutorial, you will learn how to use MySQL UNION operator to combine two or more result sets from multiple SELECT statements into a single result set. ![]() ![]() I guess this occurs after php reaches its timeout. However, if i change offset to something above 0, which is needed for the page to be paginated, the page is loading forever and eventually ends up with an almost never-ending loop, with a php notice saying 'undefined offset' for each iteration. This works just fine as long as I display it in one long list, or use 0 as offset in the LIMIT. The reason I use UNION is that the tables don't have the same columns, but I need the data to be presented together in a chronological order.
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