Query data¶

SELECT¶

The SELECT statement allows to get data from tables or views. It controls which columns and rows you want to see.

SELECT 
    column_1, column_2, ...
FROM
    table_1
[INNER | LEFT |RIGHT] JOIN table_2 ON conditions
WHERE
    conditions
GROUP BY column_1
HAVING group_conditions
ORDER BY column_1
LIMIT offset, length;

SELECT DISTINCT¶

When querying, it is possible to get duplicate rows. In order to remove duplicates, you can use DISTINCT.

SELECT DISTINCT
    columns
FROM
    table_name
WHERE
    where_conditions;

If a column has NULL value in the column, it will keep only the first occurence. When using multiple columns, it will look for unique combinations between the columns.

DISTINCT vs GROUP BY¶

If you use GROUP BY without any aggregation functions, it behaves like DISTINCT.

SELECT 
    state
FROM
    customers
GROUP BY state;

is equal to

SELECT DISTINCT
    state
FROM
    customers;

DISTINCT and aggregate function¶

You can also use DISTINCT in aggregate functions like this:

SELECT 
    COUNT(DISTINCT state)
FROM
    customers
WHERE
    country = 'USA';