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## Overview

The **`merge()` function** in R combines two data frames.

The most crucial requirement for connecting two data frames is that the column type is the same on which the merging occurs.

The `merge()` function is similar to the `join` function in a Relational Database Management System (RDMS).

## Syntax

Let's look at the syntax of the function.

```
merge(x, y, by, by.x, by.y, all.x, all.y, sort = TRUE)
```

## Arguments

* **`x`**: This is the first data frame or object to be merged.
* **`y`**: This is the second data frame or object to be merged.
* **`by`**, **`by.x`**, **`by.y`**: This specifies the columns used for the merging.
* **`all.x`**, **`all.y`**: If either is `TRUE`, additional rows will be added to the output:
   * For each row in `x` in the case of `all.x=TRUE` OR
   * For each row in `y` in the case of `all.y=TRUE`

  If the syntax does not have a matching row for the associated data frame or object, then `NA`s will be printed in these rows.
* **`sort = TRUE/FALSE`**:	This specifies if the results are sorted or not. 

## Code

Let's look at the following example:

x = data.frame(StudentId = c(1:6), 
                 Marks = c("70", "84", "90", "93", "80", "76"))

y = data.frame(StudentId = c(2, 4, 6, 7, 8), 
                 city = c("Lahore", "Karachi", "Peshawar", "Quetta", "Multan")) 

z = merge(x, y, by = "StudentId")
z

## Explanation
* Lines 1-2: We define a dataset `x` with `StudentId` and their respective `Marks`.

* Lines 4-5: We define a dataset `y` with `StudentId` and the `city` they belong to.

* Line 7: We take the natural join of the two data frames and merge the data into another dataset, `z`.

> We get three values in the output because these three values (i.e., `2`, `4`, and `6`) are common in our data frames.

How to use the merge() function for data frames in R

Combines two data frames based on common columns. Similar to SQL join. Syntax and example provided.