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-- View all clients
SELECT * FROM clients;

-- Count how many clients are in the database
SELECT COUNT(*) FROM clients;

-- Count how many clients have provided their email
SELECT COUNT(email) FROM clients;

-- View all clients orders
SELECT * FROM client_orders;

-- Calculate the total order amount across all orders
SELECT SUM(order_total) FROM client_orders;

-- Show both client count and total order value in one query
SELECT COUNT(*) AS total_clients, SUM(order_total) AS total_revenue
FROM client_orders;

# `COUNT` and `SUM`

Two of the most common SQL functions for summarizing data are `COUNT()` and `SUM()`.
They help you answer questions such as:

- How many orders were placed?
- What's the total amount spent?

<br/>

## `COUNT`: How Many Rows?

`COUNT()` returns the number of rows in a table.

```sql title="Count total clients"
SELECT COUNT(*) 
FROM clients;
```

This returns the total number of rows in the `clients` table.

To count only **non-null values** in a specific column, use `COUNT()` with the column name.

```sql title="Count clients with email"
SELECT COUNT(email)
FROM clients;
```

<br/>

## `SUM`: Add Up Values

`SUM()` adds up all values from a numeric column.

```sql title="Total sales"
SELECT SUM(order_total)
FROM client_orders;
```

This returns the total amount from all orders.

> *Note:* `SUM()` works only with numeric types such as `INT` or `REAL`.

`SUM()` is specifically designed to add up numeric values from a column, not to count rows. To count rows, you should use the `COUNT()` function, which is intended for calculating the number of entries in a table or the number of non-null values in a column.

### The SQL function `SUM()` can be used to count the number of rows in a table.

COUNT and SUM

COUNT: How Many Rows?

SUM: Add Up Values

The SQL function SUM() can be used to count the number of rows in a table.

`COUNT` and `SUM`

`COUNT`: How Many Rows?

`SUM`: Add Up Values

The SQL function `SUM()` can be used to count the number of rows in a table.