Join is the most easiest thing that could ever happen to a table, may it be a horizontal or a vertical join. Though the word vertical is not common in terms of joins but it is a common type of join that is applied in day to day SQL queries.
A horizontal join is a join specified between 2 or more object (mainly tables) based on a common element specified in the tables they may include inner joins, outer joins, cross apply, cross joins, etc.
A vertical join is combining 2 or more tables vertically using a UNION or a UNION ALL clause. All the datatype for the tables should match sequentially while joining the tables vertically using UNION or UNION ALL clause.
Example: if first table has 3 columns with datatypes Int, Varchar and datetime then the next tables used in the union or union all clause should have 3 columns with the same datatype.
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