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If you find yourself manipulating multiple SQL databases and tables at the same time, it’s standard to combine various SQL queries into one result set. This approach simplifies your task and increases the efficiency and effectiveness of your data processes. Here, we will discuss two common SQL functions used to combine multiple queries into one simple result set. They are UNION and UNION ALL.
UNION Function
The UNION operation in SQL is used to combine the results of two or more SELECT statements without returning any duplicate rows. To use UNION, each SELECT statement needs to have the same number of columns. The columns also must have similar data types.
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SELECT column_name1, column_name2, ... FROM table_name1 UNION SELECT column_name1, column_name2, ... FROM table_name2; |
Keep in mind, the UNION operator selects only distinct values by default.
UNION ALL Function
UNION ALL function performs the same operation as UNION, but it does include duplicate rows in the result set. It may be useful in cases where the duplication of rows is necessary.
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SELECT column_name1, column_name2, ... FROM table_name1 UNION ALL SELECT column_name1, column_name2, ... FROM table_name2; |
Remember, UNION ALL does not remove duplicate rows, so you may end up with duplicate entries in the result set if they are presented in the source tables.
Combining UNION or UNION ALL with Other Statements
You can also combine UNION or UNION ALL with other SQL statements like WHERE, ORDER BY, LIMIT, etc. See below a SQL query that combines the UNION operator with a WHERE statement:
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SELECT column_name1, column_name2, ... FROM table_name1 WHERE condition UNION SELECT column_name1, column_name2, ... FROM table_name2 WHERE condition; |
In conclusion, UNION and UNION ALL operators are powerful tools to combine multiple queries into a single result set. Use them wisely and enhance your SQL querying skills.