SQL Resources/SQL/Understanding WHERE# Understanding WHERE

The WHERE clause is one of the fundamental building blocks of a SQL SELECT statement. In this article we'll explain how it works.

`-- Generic SQL SELECT statement select -- (some columns) from -- (some tables) where -- (A 'predicate' expression to be used as a filter) group by -- (some columns)`

The WHERE clause is formed of the reserved 'where' keyword followed by what is known as a predicate expression - simply an expression which returns true or false.

If, for a given row, the predicate returns true, that row will be included in the output of the query.

If the predicate returns false, that row will be excluded.

Let's look at some examples.

In these examples, the predicate expression in the WHERE clause is a simple boolean expression - an expression that returns true or false.

```
with my_table as (select * from unnest([1,2,3,4,5]) as numbers)
select numbers from my_table
where
numbers > 2 -- This is the predicate expression
```

numbers

3

4

5

```
with my_table as (select * from unnest([1,2,3,4,5]) as numbers)
select numbers from my_table
where
numbers = 2
```

numbers

2

```
with my_table as (select * from unnest([null,2,3,4,null]) as numbers)
select numbers from my_table
where
numbers is not null
```

numbers

2

3

4

```
with my_table as (select * from unnest([null,2,3,4,null]) as numbers)
select numbers from my_table
where
numbers is null
```

numbers

NULL

NULL

```
with my_table as (select * from unnest([1,2,3,4,5]) as numbers)
select numbers from my_table
where
numbers in (1,2)
```

numbers

1

2

```
with my_table as (select * from unnest([1,2,3,4,5]) as numbers)
select numbers from my_table
where
numbers between 2 and 4 -- Note - these limits are inclusive
```

numbers

2

3

4

In the following examples, the WHERE clause contains more complex predicate expressions, using the building blocks of the AND and OR logical operators.

```
with my_table as (select * from unnest([1,2,3,4,5]) as numbers)
select numbers from my_table
where
-- Use brackets to control precedence
numbers > 2 and (numbers < 3 or numbers > 4)
```

numbers

5

```
with my_table as (select * from unnest(['a', 'b', 'ab', 'aa', 'bb', 'cc']) as strings)
select strings from my_table
where
-- Predicate expressions can contain functions
(strings like 'a%' or strings like 'b%') and length(strings) = 2
```

strings

ab

aa

bb

```
with my_table as (select * from unnest([1,2,3,4,5]) as numbers)
select numbers from my_table
where
-- Predicate expressions can contain other select statements
numbers > (select 1+1)
```

numbers

3

4

5

When using dates and times, all of the same rules apply. The predicate expression in the WHERE clause must still return true or false, but you are free to use functions and compound logical expressions.

```
with my_table as (
select * from unnest(generate_date_array('2020-01-01', '2020-01-10')) as dates
)
select dates from my_table
where
dates = '2020-01-01'
```

dates

2020-01-01

```
with my_table as (
select * from unnest(generate_date_array('2020-01-01', '2020-01-10')) as dates
)
select dates from my_table
where
dates > '2020-01-05'
```

dates

2020-01-06

2020-01-07

2020-01-08

2020-01-09

2020-01-10

```
with my_table as (
select * from unnest(generate_date_array('2020-01-01', '2020-01-10')) as dates
)
select dates from my_table
where
-- Note - between is inclusive
dates between '2020-01-02' and '2020-01-04'
```

dates

2020-01-02

2020-01-03

2020-01-04

```
with my_table as (
select * from unnest(generate_date_array('2020-01-01', '2020-01-10')) as dates
)
select dates from my_table
where
extract(day from dates) = 5 and extract(month from dates) = 1
```

dates

2020-01-05

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