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43UNION_UNIONALL.sql

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+create table employees_dup(
+emp_no int,
+birth_date date,
+first_name varchar(40),
+last_name varchar(40),
+gender varchar(40),
+hire_date date
+);
+
+insert into employees_dup
+select * from employees
+limit 10;
+
+delete from employees_dup;
+
+insert into employees_dup
+select * from employees
+limit 20;
+
+insert into employees_dup
+values(10001,'1953-09-02','Georgi','Facello','M','1986-06-26');
+
+#UNION and UNION_ALL
+SELECT 
+    e.emp_no,
+    e.first_name,
+    e.last_name,
+    NULL AS dept_no,
+    NULL AS from_date
+FROM
+    employees_dup e
+WHERE
+    e.emp_no = 10001 
+UNION  SELECT #for union, just type UNION instead of UNION ALL
+    NULL AS emp_no,
+    NULL AS first_name,
+    NULL AS last_name,
+    m.dept_no,
+    m.from_date
+FROM
+    dept_manager m;
+#note:- UNION is similar to JOIN but not the same. UNION adds everything in the tables whereas JOINS give only the matching values unless there is a condition(like left or right join).
+
+#alt_def:-OIN, depending on whether INNER, LEFT, or RIGHT, allows you to obtain output that joins records from various tables in a specific way. You can think of it as horizontally combining records from various tables.
+#UNION, instead, adds, or, better, unifies data from different tables. However, it is important that these tables are of the same length and contain the same column names.
+#Here, the records are combined vertically, so you can think of the first table being vertically extended by the content of the other table(s) you are unifying it with.
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