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MySQL - UNION vs UNION ALL
UNION and UNION ALL operators in MySQL are used to retrieve the rows from multiple tables and return them as one single table.
However, for these operators to work on these tables, they need to follow the conditions given below −
- The tables to be combined must have the same number of columns with the same datatype.
- The number of rows need not be the same.
MySQL UNION Operator
UNION operator in MySQL works similar to the union operator in relational algebra. This operator combines information from multiple tables that are union compatible. However, only distinct rows from the tables are added to the result table, as UNION automatically eliminates all the duplicate records.
Syntax
Following is the syntax of UNION operator in MySQL −
SELECT * FROM table1 UNION SELECT * FROM table2;
MySQL UNION ALL Operator
UNION ALL is also an operator/clause in MySQL, used to combine multiple tables into one. However, this operator also preserves the duplicate rows in the resultant tables.
Syntax
Following is the syntax of UNION ALL operator in MySQL −
SELECT * FROM table1 UNION ALL SELECT * FROM table2;
MySQL UNION VS MySQL UNION ALL
The only difference between these two operators is that UNION only returns distinct rows while UNION ALL returns all the rows present in both tables. Let's try to understand this with an example.Example
First of all we need to create two tables namely with the same number of columns having the same data types. Following query creates a table named "COURSES_PICKED" −
CREATE TABLE COURSES_PICKED( STUDENT_ID INT NOT NULL, STUDENT_NAME VARCHAR(30) NOT NULL, COURSE_NAME VARCHAR(30) NOT NULL );
Following query inserts values into the COURSES_PICKED table −
INSERT INTO COURSES_PICKED VALUES (1, 'JOHN', 'ENGLISH'), (2, 'ROBERT', 'COMPUTER SCIENCE'), (3, 'SASHA', 'COMMUNICATIONS'), (4, 'JULIAN', 'MATHEMATICS');
The contents of the COURSES_PICKED table will be as follows −
STUDENT_ID | STUDENT_NAME | COURSE_NAME |
---|---|---|
1 | JOHN | ENGLISH |
2 | ROBERT | COMPUTER SCIENCE |
3 | SASHA | COMMUNICATIONS |
4 | JULIAN | MATHEMATICS |
Now, lets create another table named EXTRA_COURSES_PICKED as shown below −
CREATE TABLE EXTRA_COURSES_PICKED( STUDENT_ID INT NOT NULL, STUDENT_NAME VARCHAR(30) NOT NULL, EXTRA_COURSE_NAME VARCHAR(30) NOT NULL );
Following is the query to insert values into the EXTRA_COURSES_PICKED table −
INSERT INTO EXTRA_COURSES_PICKED VALUES (1, 'JOHN', 'PHYSICAL EDUCATION'), (2, 'ROBERT', 'GYM'), (3, 'SASHA', 'FILM'), (4, 'JULIAN', 'MATHEMATICS');
The contents of the EXTRA_COURSES_PICKED table will be as follows −
STUDENT_ID | STUDENT_NAME | COURSES_PICKED |
---|---|---|
1 | JOHN | PHYSICAL EDUCATION |
2 | ROBERT | GYM |
3 | SASHA | COMMUNICATIONS |
4 | JULIAN | MATHEMATICS |
Combining tables using UNION Operator
Now, let us combine both these tables using the UNION query as follows −
SELECT * FROM COURSES_PICKED UNION SELECT * FROM EXTRA_COURSES_PICKED;
Output
Following is the output obtained −
STUDENT_ID | STUDENT_NAME | COURSE_NAME |
---|---|---|
1 | JOHN | ENGLISH |
2 | ROBERT | COMPUTER SCIENCE |
3 | SASHA | COMMUNICATIONS |
4 | JULIAN | MATHEMATICS |
1 | JOHN | PHYSICAL EDUCATION |
2 | ROBERT | GYM |
In here, the extra courses and courses picked by "Julian" and "Shasha" are same therefore, the last two records in the COURSES_PICKED and EXTRA_COURSES_PICKED are identical. When we combine both the tables using the UNION operator it excludes the identical records and returns only distinct records.
Combining tables using UNION ALL Operator
Unlike the UNION operator if we combine the above created tables using the UNION ALL operator it reruns all the records in both tables −
SELECT * FROM COURSES_PICKED UNION ALL SELECT * FROM EXTRA_COURSES_PICKED;
Output
The resultant table is displayed as follows −
STUDENT_ID | STUDENT_NAME | COURSE_NAME |
---|---|---|
1 | JOHN | ENGLISH |
2 | ROBERT | COMPUTER SCIENCE |
3 | SASHA | COMMUNICATIONS |
4 | JULIAN | MATHEMATICS |
1 | JOHN | PHYSICAL EDUCATION |
2 | ROBERT | GYM |
3 | SASHA | COMMUNICATIONS |
4 | JULIAN | MATHEMATICS |
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