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SELECT Clause

The SELECT clause specifies the list of columns that will be returned by the query. While it appears first in the clause, logically the expressions here are executed only at the end. The SELECT clause can contain arbitrary expressions that transform the output, as well as aggregates and window functions.

Examples

-- select all columns from the table called "table_name"
SELECT * FROM table_name;
-- perform arithmetic on columns in a table, and provide an alias
SELECT col1 + col2 AS res, sqrt(col1) AS root FROM table_name;
-- select all unique cities from the addresses table
SELECT DISTINCT city FROM addresses;
-- return the total number of rows in the addresses table
SELECT count(*) FROM addresses;
-- select all columns except the city column from the addresses table
SELECT * EXCLUDE (city) FROM addresses;
-- select all columns from the addresses table, but replace city with lower(city)
SELECT * REPLACE (lower(city) AS city) FROM addresses;
-- select all columns matching the given regex from the table
SELECT COLUMNS('number\d+') FROM addresses;
-- compute a function on all given columns of a table
SELECT min(COLUMNS(*)) FROM addresses;
-- to select columns with spaces or special characters, use double quotes
SELECT "Some Column Name" FROM tbl;

Syntax

SELECT List

The SELECT clause contains a list of expressions that specify the result of a query. The select list can refer to any columns in the FROM clause, and combine them using expressions. As the output of a SQL query is a table - every expression in the SELECT clause also has a name. The expressions can be explicitly named using the AS clause (e.g., expr AS name). If a name is not provided by the user the expressions are named automatically by the system.

Column names are case-insensitive. See the Rules for Case Sensitivity for more details.

Star Expressions

-- select all columns from the table called "table_name"
SELECT *
FROM table_name;
-- select all columns matching the given regex from the table
SELECT COLUMNS('number\d+')
FROM addresses;

The star expression is a special expression that expands to multiple expressions based on the contents of the FROM clause. In the simplest case, * expands to all expressions in the FROM clause. Columns can also be selected using regular expressions or lambda functions. See the star expression page for more details.

DISTINCT Clause

-- select all unique cities from the addresses table
SELECT DISTINCT city
FROM addresses;

The DISTINCT clause can be used to return only the unique rows in the result - so that any duplicate rows are filtered out.

Queries starting with SELECT DISTINCT run deduplication, which is an expensive operation. Therefore, only use DISTINCT if necessary.

DISTINCT ON Clause

-- select only the highest population city for each country
SELECT DISTINCT ON(country) city, population
FROM cities
ORDER BY population DESC;

The DISTINCT ON clause returns only one row per unique value in the set of expressions as defined in the ON clause. If an ORDER BY clause is present, the row that is returned is the first row that is encountered as per the ORDER BY criteria. If an ORDER BY clause is not present, the first row that is encountered is not defined and can be any row in the table.

When querying large data sets, using DISTINCT on all columns can be expensive. Therefore, consider using DISTINCT ON on a column (or a set of columns) which guaranetees a sufficient degree of uniqueness for your results. For example, using DISTINCT ON on the key column(s) of a table guarantees full uniqueness.

Aggregates

-- return the total number of rows in the addresses table
SELECT count(*)
FROM addresses;
-- return the total number of rows in the addresses table grouped by city
SELECT city, count(*)
FROM addresses
GROUP BY city;

Aggregate functions are special functions that combine multiple rows into a single value. When aggregate functions are present in the SELECT clause, the query is turned into an aggregate query. In an aggregate query, all expressions must either be part of an aggregate function, or part of a group (as specified by the GROUP BY clause).

Window Functions

-- generate a "row_number" column containing incremental identifiers for each row
SELECT row_number() OVER ()
FROM sales;
-- compute the difference between the current amount, and the previous amount, by order of time
SELECT amount - lag(amount) OVER (ORDER BY time)
FROM sales;

Window functions are special functions that allow the computation of values relative to other rows in a result. Window functions are marked by the OVER clause which contains the window specification. The window specification defines the frame or context in which the window function is computed. See the window functions page for more information.

unnest Function

-- unnest an array by one level
SELECT unnest([1, 2, 3]);
-- unnest a struct by one level
SELECT unnest({'a': 42, 'b': 84});

The unnest function is a special function that can be used together with arrays, lists, or structs. The unnest function strips one level of nesting from the type. For example, INT[] is transformed into INT. STRUCT(a INT, b INT) is transformed into a INT, b INT. The unnest function can be used to transform nested types into regular scalar types, which makes them easier to operate on.

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Last modified: 2024-03-02