# Logical expressions

Logical expressions combine conditions. SQL uses three truth values: true, false and unknown, represented by NULL.

## Syntax

```text
condition AND condition
condition OR condition
NOT condition
```

Operands are converted using SQL truth rules. A false operand makes AND false even when the other operand is NULL. A true operand makes OR true even when the other operand is NULL. NOT NULL remains NULL.

## Truth table

| A     | B     | A AND B | A OR B |
| ----- | ----- | ------- | ------ |
| true  | true  | true    | true   |
| true  | false | false   | true   |
| true  | NULL  | NULL    | true   |
| false | false | false   | false  |
| false | NULL  | false   | NULL   |
| NULL  | NULL  | NULL    | NULL   |

## Examples

### Combining missing and known conditions

```sql
SELECT 1 AND NULL, 0 AND NULL, 1 OR NULL, NOT 0
FORMAT csv;
```

```text
\N,0,1,1
```

### Selecting matching rows

```sql
CREATE TABLE orders (id INTEGER, paid Bool, amount INTEGER);
INSERT INTO orders VALUES (1, true, 100), (2, false, 200), (3, true, 40);
```

```sql
SELECT id
FROM orders
WHERE paid AND amount >= 50
ORDER BY id
FORMAT csv;
```

```text
1
```

A WHERE condition admits only true, so both false and NULL remove a row. Use `IS NULL` to test for a missing value, and [conditional functions](https://coordinatordb.com/docs/reference/functions/regular-functions/conditional-functions) or [coalesce](https://coordinatordb.com/docs/reference/functions/regular-functions/functions-for-nulls#coalesce) to choose a fallback.