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SQL expressions are an essential part of writing optimal code, allowing practitioners to formulate complex queries that filter, group, and aggregate data. Using expressions is critical for efficient querying and manipulating data to deliver compelling insights. Most SQL that is written in everyday practice uses some kind of SQL expression and offers an opportunity for the author to practice logical ways of manipulating data.

## What are SQL Expressions?

SQL expressions are statements that are used in code to perform calculations, comparisons, and logical operations on data stored in tables and databases. Using expressions introduces the opportunity to think creatively about a data output and is common in exploratory data analysis, dimensional cuts and segmentation work.

SQL expressions can be used in various parts of a SQL query, including SELECT, WHERE, GROUP BY, HAVING, and ORDER BY clauses.

Some common examples of SQL expressions are:

Arithmetic Expressions:

```SELECT
revenue_in_cents/100AS revenue_in_usd
FROM sales_table
```

In the above sales_table, the revenue_in_cents column isn’t particularly valuable on its own. By using an arithmetic SQL expression to divide the revenue_in_cents by 100, the column converts to dollars, making the insights more intuitive. Renaming the column using a SQL alias makes the output data more readable.

Logical Expressions:

```SELECT
country,
COUNT(orders)
FROM orders_table
WHERE price >100
AND region = ‘APAC’
GROUP BY country
```

In the above example, the WHERE expression is being used to filter the data in the orders_table in order to only show results where the price is >100 and the region = APAC.

## Why are SQL Expressions Important?

As exhibited in the examples above, data queried directly from a table with no filters or expressions can be confusing to interpret and generally suboptimal.  With SQL expressions, data becomes much more applicable in practice. Column values can be calculated, filtered and ordered in a few seconds by leveraging the right SQL expression.

A few valuable use cases that highlight the importance of SQL expressions are:

1. Improving Data Analysis: Leveraging expressions allows for analyzing and manipulating data in ways that are not possible otherwise. They allow for filtering on column values and dates, while also running simple or even complex arithmetic on aggregate metric values.
2. Query Optimization:  Expressions improve efficiency by reducing the amount of data needed to be retrieved from a database. They allow the data practitioner to specifically define only the necessary data, reducing the number of records that need to be processed.
3. Data Accuracy:  SQL expressions can be used to validate data and ensure that it meets specific criteria. For example, during querying, a data practitioner can use SQL expressions to ensure that data is within a certain range, or that it matches a specific format.

## How to Use SQL Expressions

SQL expressions can be used in various parts of an SQL query depending on the desired data output. Here are some examples of how SQL expressions can be used in practice across a variety of data aggregation needs:

CONCAT Clause

```SELECT
CONCAT(first_name,’ ‘, last_name) AS full_name,
(price * quantity) AS total_price
FROM orders_table
```

In the above example, the CONCAT  function is used to concatenate the first_name and last_name string columns in the orders table. The AS keyword is used to give the resulting column an alias (full_name). The multiplication operator (*) is used to calculate the total_price column by multiplying the price and quantity columns in the orders table.

GROUP BY Clause

The GROUP BY clause is used to group data based on specific columns, particularly when aggregations are being used.

```SELECT
region,
year,
AVG(total_price) AS average_price
FROM sales_table
GROUP BY region, year
```

In the above example, the GROUP BY clause groups the average_price calculation by region and year.

Date and Time Expressions

Date and time expressions in SQL are not only useful, but often necessary since most data has some kind of time component. There is an expansive list of expressions in SQL that deal with date and time data. Here are some of the most common:

Using dates as a filter with a WHERE clause as seen below:

```SELECT
COUNT(DISTINCT order_id) AS num_orders
FROM orders_table
WHERE order_date >= 2022-01-01
```

In the above, this query only returns the number of orders that were placed since 2022-01-01.

CURRENT_DATE: This expression returns the current date in the database’s time zone. It is useful when querying data BETWEEN a starting_date and the CURRENT_DATE.

CURRENT_TIME: This expression returns the current time in the database’s time zone.

DATEADD: This expression adds a specified number of intervals (such as days, months, or years) to a date or time value. Let’s say you have a table called appointments with columns appointment_id, appointment_start_time, and duration_minutes. You want to retrieve a list of appointment start and end times, where the end time is the start time plus the duration of the appointment in minutes.

```SELECT
appointment_id,
appointment_date,
duration_minutes,
FROM appointments
```

Using the DATEADD expression, the appointment_end_time is simple to calculate.

DATEDIFF: This expression calculates the difference between two date or time values in a specified interval (such as days, months, or years).

For example, let’s say there is a need to quickly see the ages of all users in a table called users_table. The DATEDIFF expression can be used to calculate the age of a user by using the birth_date column and the CURRENT_DATE expression.

```SELECT
first_name,
Last_name,
DATEDIFF(year, birth_date, CURRENT_DATE()) AS age_in_years
FROM users_table
```

## Conclusion

From simple arithmetic operations to more complex functions like DATEADD or DATEDIFF, SQL expressions create optimal data outputs by transforming and deriving insights from a datasource. By mastering SQL expressions, it becomes easier to write efficient queries and gain valuable insights, ultimately making better decisions with data.

Looking to expand your knowledge of SQL? Take our SQL online Nanodegree program to gain in-demand skills and master the core programming language for data analysis.

Emily Glauser