Union (∪)

Union (∪)

The UNION operation combines the selected rows from two tables into a single result set. When you apply UNION, every row that appears in either of the two tables is included in the output. A key rule: if the same row exists in both tables, it appears only once in the final result — duplicates are automatically removed.

Think of it like taking two sets and merging them into one. For example, if one set contains students in Dance and another contains students in Music, their union gives you the complete list of students who participate in at least one of the two events. The book illustrates this with a Venn diagram (Figure 9.4) showing two overlapping circles labelled "Dance" and "Music", where the shaded area covering both circles represents the union.

Example with DANCE and MUSIC tables

Consider two relations: DANCE and MUSIC, each storing student participation data with columns SNo, Name, and Class.

Table 9.18: DANCE

Table 9.19: MUSIC

Notice that Mahira (SNo 2) and Sanjay (SNo 4) appear in both tables. When we apply the UNION operation (written as DANCE ∪ MUSIC), these duplicate rows are listed only once.

Table 9.20: DANCE ∪ MUSIC

The result has seven rows — the four from DANCE plus the three from MUSIC that were not already present (Mehak, Lavanya, Abhay). Mahira and Sanjay appear only once, even though they were in both original tables. …