Select (SQL) explained

The SQL SELECT statement returns a result set of rows, from one or more tables.^{[1] [2]}

A SELECT statement retrieves zero or more rows from one or more database tables or database views. In most applications, SELECT is the most commonly used data manipulation language (DML) command. As SQL is a declarative programming language, SELECT queries specify a result set, but do not specify how to calculate it. The database translates the query into a "query plan" which may vary between executions, database versions and database software. This functionality is called the "query optimizer" as it is responsible for finding the best possible execution plan for the query, within applicable constraints.

The SELECT statement has many optional clauses:

• SELECT list is the list of columns or SQL expressions to be returned by the query. This is approximately the relational algebra projection operation.
• [[Alias (SQL)|AS]] optionally provides an alias for each column or expression in the SELECT list. This is the relational algebra rename operation.
• [[From (SQL)|FROM]] specifies from which table to get the data.^[3]
• [[Where (SQL)|WHERE]] specifies which rows to retrieve. This is approximately the relational algebra selection operation.
• [[Group by (SQL)|GROUP BY]] groups rows sharing a property so that an aggregate function can be applied to each group.
• [[Having (SQL)|HAVING]] selects among the groups defined by the GROUP BY clause.
• [[Order by (SQL)|ORDER BY]] specifies how to order the returned rows.

Overview

SELECT is the most common operation in SQL, called "the query". SELECT retrieves data from one or more tables, or expressions. Standard SELECT statements have no persistent effects on the database. Some non-standard implementations of SELECT can have persistent effects, such as the SELECT INTO syntax provided in some databases.^[4]

Queries allow the user to describe desired data, leaving the database management system (DBMS) to carry out planning, optimizing, and performing the physical operations necessary to produce that result as it chooses.

A query includes a list of columns to include in the final result, normally immediately following the SELECT keyword. An asterisk ("*") can be used to specify that the query should return all columns of all the queried tables. SELECT is the most complex statement in SQL, with optional keywords and clauses that include:

• The [[From (SQL)|FROM]] clause, which indicates the table(s) to retrieve data from. The FROM clause can include optional [[Join (SQL)|JOIN]] subclauses to specify the rules for joining tables.
• The [[Where (SQL)|WHERE]] clause includes a comparison predicate, which restricts the rows returned by the query. The WHERE clause eliminates all rows from the result set where the comparison predicate does not evaluate to True.
• The GROUP BY clause projects rows having common values into a smaller set of rows. GROUP BY is often used in conjunction with SQL aggregation functions or to eliminate duplicate rows from a result set. The WHERE clause is applied before the GROUP BY clause.
• The [[Having (SQL)|HAVING]] clause includes a predicate used to filter rows resulting from the GROUP BY clause. Because it acts on the results of the GROUP BY clause, aggregation functions can be used in the HAVING clause predicate.
• The [[Order by (SQL)|ORDER BY]] clause identifies which column[s] to use to sort the resulting data, and in which direction to sort them (ascending or descending). Without an ORDER BY clause, the order of rows returned by an SQL query is undefined.
• The DISTINCT keyword^[5] eliminates duplicate data.^[6]

The following example of a SELECT query returns a list of expensive books. The query retrieves all rows from the Book table in which the price column contains a value greater than 100.00. The result is sorted in ascending order by title. The asterisk (*) in the select list indicates that all columns of the Book table should be included in the result set.

SELECT * FROM Book WHERE price > 100.00 ORDER BY title;

The example below demonstrates a query of multiple tables, grouping, and aggregation, by returning a list of books and the number of authors associated with each book.

SELECT Book.title AS Title, count(*) AS Authors FROM Book JOIN Book_author ON Book.isbn = Book_author.isbn GROUP BY Book.title;

Example output might resemble the following:

Title Authors ---------------------- ------- SQL Examples and Guide 4 The Joy of SQL 1 An Introduction to SQL 2 Pitfalls of SQL 1

Under the precondition that isbn is the only common column name of the two tables and that a column named title only exists in the Book table, one could re-write the query above in the following form:

SELECT title, count(*) AS Authors FROM Book NATURAL JOIN Book_author GROUP BY title;

However, many vendors either do not support this approach, or require certain column-naming conventions for natural joins to work effectively.

SQL includes operators and functions for calculating values on stored values. SQL allows the use of expressions in the select list to project data, as in the following example, which returns a list of books that cost more than 100.00 with an additional sales_tax column containing a sales tax figure calculated at 6% of the price.

SELECT isbn, title, price, price * 0.06 AS sales_tax FROM Book WHERE price > 100.00 ORDER BY title;

Subqueries

Queries can be nested so that the results of one query can be used in another query via a relational operator or aggregation function. A nested query is also known as a subquery. While joins and other table operations provide computationally superior (i.e. faster) alternatives in many cases (all depending on implementation), the use of subqueries introduces a hierarchy in execution that can be useful or necessary. In the following example, the aggregation function AVG receives as input the result of a subquery:

SELECT isbn, title, price FROM Book WHERE price < (SELECT AVG(price) FROM Book) ORDER BY title;

A subquery can use values from the outer query, in which case it is known as a correlated subquery.

Since 1999 the SQL standard allows WITH clauses, i.e. named subqueries often called common table expressions (named and designed after the IBM DB2 version 2 implementation; Oracle calls these subquery factoring). CTEs can also be recursive by referring to themselves; the resulting mechanism allows tree or graph traversals (when represented as relations), and more generally fixpoint computations.

Derived table

A derived table is a subquery in a FROM clause. Essentially, the derived table is a subquery that can be selected from or joined to. Derived table functionality allows the user to reference the subquery as a table. The derived table also is referred to as an inline view or a select in from list.

In the following example, the SQL statement involves a join from the initial Books table to the derived table "Sales". This derived table captures associated book sales information using the ISBN to join to the Books table. As a result, the derived table provides the result set with additional columns (the number of items sold and the company that sold the books):

SELECT b.isbn, b.title, b.price, sales.items_sold, sales.company_nmFROM Book b JOIN (SELECT SUM(Items_Sold) Items_Sold, Company_Nm, ISBN FROM Book_Sales GROUP BY Company_Nm, ISBN) sales ON sales.isbn = b.isbn

Examples

Table "T"	Query	Result
C1 C2 1 a 2 b		C1 C2 1 a 2 b
C1 C2 1 a 2 b		C1 1 2
C1 C2 1 a 2 b
C1 C2 1 a 2 b		C1 C2 2 b 1 a
does not exist

Given a table T, the query will result in all the elements of all the rows of the table being shown.

With the same table, the query will result in the elements from the column C1 of all the rows of the table being shown. This is similar to a projection in relational algebra, except that in the general case, the result may contain duplicate rows. This is also known as a Vertical Partition in some database terms, restricting query output to view only specified fields or columns.

With the same table, the query will result in all the elements of all the rows where the value of column C1 is '1' being shown in relational algebra terms, a selection will be performed, because of the WHERE clause. This is also known as a Horizontal Partition, restricting rows output by a query according to specified conditions.

With more than one table, the result set will be every combination of rows. So if two tables are T1 and T2, will result in every combination of T1 rows with every T2 rows. E.g., if T1 has 3 rows and T2 has 5 rows, then 15 rows will result.

Although not in standard, most DBMS allows using a select clause without a table by pretending that an imaginary table with one row is used. This is mainly used to perform calculations where a table is not needed.

The SELECT clause specifies a list of properties (columns) by name, or the wildcard character (“*”) to mean “all properties”.

Limiting result rows

Often it is convenient to indicate a maximum number of rows that are returned. This can be used for testing or to prevent consuming excessive resources if the query returns more information than expected. The approach to do this often varies per vendor.

In ISO, result sets may be limited by using

• cursors, or
• by adding a SQL window function to the SELECT-statement

ISO introduced the FETCH FIRST clause.

According to PostgreSQL v.9 documentation, an SQL window function "performs a calculation across a set of table rows that are somehow related to the current row", in a way similar to aggregate functions.^[7] The name recalls signal processing window functions. A window function call always contains an OVER clause.

ROW_NUMBER window function

ROW_NUMBER OVER may be used for a simple table on the returned rows, e.g. to return no more than ten rows:

SELECT * FROM(SELECT ROW_NUMBER OVER (ORDER BY sort_key ASC) AS row_number, columns FROM tablename) AS fooWHERE row_number <= 10

ROW_NUMBER can be non-deterministic: if sort_key is not unique, each time you run the query it is possible to get different row numbers assigned to any rows where sort_key is the same. When sort_key is unique, each row will always get a unique row number.

RANK window function

The RANK OVER window function acts like ROW_NUMBER, but may return more or less than n rows in case of tie conditions, e.g. to return the top-10 youngest persons:

SELECT * FROM (SELECT RANK OVER (ORDER BY age ASC) AS ranking, person_id, person_name, age FROM person) AS fooWHERE ranking <= 10

The above code could return more than ten rows, e.g. if there are two people of the same age, it could return eleven rows.

FETCH FIRST clause

Since ISO results limits can be specified as in the following example using the FETCH FIRST clause.

SELECT * FROM T FETCH FIRST 10 ROWS ONLY

This clause currently is supported by CA DATACOM/DB 11, IBM DB2, SAP SQL Anywhere, PostgreSQL, EffiProz, H2, HSQLDB version 2.0, Oracle 12c and Mimer SQL.

Microsoft SQL Server 2008 and higher supports FETCH FIRST, but it is considered part of the ORDER BY clause. The ORDER BY, OFFSET, and FETCH FIRST clauses are all required for this usage.

SELECT * FROM T ORDER BY acolumn DESC OFFSET 0 ROWS FETCH FIRST 10 ROWS ONLY

Non-standard syntax

Some DBMSs offer non-standard syntax either instead of or in addition to SQL standard syntax. Below, variants of the simple limit query for different DBMSes are listed:

SET ROWCOUNT 10SELECT * FROM T	MS SQL Server (This also works on Microsoft SQL Server 6.5 while the Select top 10 * from T does not)
SELECT * FROM T LIMIT 10 OFFSET 20	Netezza, MySQL, MariaDB (also supports the standard version, since version 10.6), SAP SQL Anywhere, PostgreSQL (also supports the standard, since version 8.4), SQLite, HSQLDB, H2, Vertica, Polyhedra, Couchbase Server, Snowflake Computing, OpenLink Virtuoso
SELECT * from T WHERE ROWNUM <= 10	Oracle
SELECT '''FIRST 10''' * from T	Ingres
SELECT '''FIRST 10''' * FROM T order by a	Informix
SELECT '''SKIP 20 FIRST 10''' * FROM T order by c, d	Informix (row numbers are filtered after order by is evaluated. SKIP clause was introduced in a v10.00.xC4 fixpack)
SELECT '''TOP 10''' * FROM T	MS SQL Server, SAP ASE, MS Access, SAP IQ, Teradata
SELECT * FROM T SAMPLE 10	Teradata
SELECT '''TOP 20, 10''' * FROM T	OpenLink Virtuoso (skips 20, delivers next 10)^[8]
SELECT '''TOP 10 START AT 20''' * FROM T	SAP SQL Anywhere (also supports the standard, since version 9.0.1)
SELECT '''FIRST 10 SKIP 20''' * FROM T	Firebird
SELECT * FROM TROWS 20 TO 30	Firebird (since version 2.1)
SELECT * FROM TWHERE ID_T > 10 FETCH FIRST 10 ROWS ONLY	IBM Db2
SELECT * FROM TWHERE ID_T > 20 FETCH FIRST 10 ROWS ONLY	IBM Db2 (new rows are filtered after comparing with key column of table T)

Rows Pagination

Rows Pagination^[9] is an approach used to limit and display only a part of the total data of a query in the database. Instead of showing hundreds or thousands of rows at the same time, the server is requested only one page (a limited set of rows, per example only 10 rows), and the user starts navigating by requesting the next page, and then the next one, and so on. It is very useful, specially in web systems, where there is no dedicated connection between the client and the server, so the client does not have to wait to read and display all the rows of the server.

Data in Pagination approach

• {rows} = Number of rows in a page
• {page_number} = Number of the current page
• {begin_base_0} = Number of the row - 1 where the page starts = (page_number-1) * rows

Simplest method (but very inefficient)

1. Select all rows from the database
2. Read all rows but send to display only when the row_number of the rows read is between {begin_base_0 + 1} and {begin_base_0 + rows}

Select * from order by

Other simple method (a little more efficient than read all rows)

1. Select all the rows from the beginning of the table to the last row to display ({begin_base_0 + rows})
2. Read the {begin_base_0 + rows} rows but send to display only when the row_number of the rows read is greater than {begin_base_0}

SQL	Dialect
select *from order by FETCH FIRST ROWS ONLY	SQL ANSI 2008 PostgreSQL SQL Server 2012 Derby Oracle 12c DB2 12 Mimer SQL
Select *from order by LIMIT	MySQL SQLite
Select TOP * from order by	SQL Server 2005
Select *from order by ROWS LIMIT	Sybase, ASE 16 SP2
SET ROWCOUNT Select * from order by SET ROWCOUNT 0	Sybase, SQL Server 2000
Select * FROM (SELECT * FROM ORDER BY) a where rownum <=	Oracle 11

Method with positioning

1. Select only {rows} rows starting from the next row to display ({begin_base_0 + 1})
2. Read and send to display all the rows read from the database

SQL	Dialect
Select *from order by OFFSET ROWSFETCH NEXT ROWS ONLY	SQL ANSI 2008 PostgreSQL SQL Server 2012 Derby Oracle 12c DB2 12 Mimer SQL
Select *from order by LIMIT OFFSET	MySQL MariaDB PostgreSQL SQLite
Select * from order by LIMIT,	MySQL MariaDB SQLite
Select *from order by ROWS LIMIT OFFSET	Sybase, ASE 16 SP2
Select TOP *, _offset=identity(10) into #tempfrom ORDER BY select * from #temp where _offset > DROP TABLE #temp	Sybase 12.5.3:
SET ROWCOUNT select *, _offset=identity(10) into #tempfrom ORDER BY select * from #temp where _offset > DROP TABLE #tempSET ROWCOUNT 0	Sybase 12.5.2:
select TOP * from (select *, ROW_NUMBER over (order by) as _offset from) xx where _offset >	SQL Server 2005
SET ROWCOUNT select *, _offset=identity(int,1,1) into #tempfrom ORDER BY select * from #temp where _offset > DROP TABLE #tempSET ROWCOUNT 0	SQL Server 2000
SELECT * FROM (SELECT rownum-1 as _offset, a.* FROM(SELECT * FROM ORDER BY) a WHERE rownum <=)WHERE _offset >=	Oracle 11

Method with filter (it is more sophisticated but necessary for very big dataset)

1. Select only then {rows} rows with filter:
   1. First Page: select only the first {rows} rows, depending on the type of database
   2. Next Page: select only the first {rows} rows, depending on the type of database, where the {unique_key} is greater than {last_val} (the value of the {unique_key} of the last row in the current page)
   3. Previous Page: sort the data in the reverse order, select only the first {rows} rows, where the {unique_key} is less than {first_val} (the value of the {unique_key} of the first row in the current page), and sort the result in the correct order
2. Read and send to display all the rows read from the database

First Page	Next Page	Previous Page	Dialect
select *from order by FETCH FIRST ROWS ONLY	select * from where > order by FETCH FIRST ROWS ONLY	select * from (select * from where < order by DESC FETCH FIRST ROWS ONLY) a order by	SQL ANSI 2008 PostgreSQL SQL Server 2012 Derby Oracle 12c DB2 12 Mimer SQL
select *from order by LIMIT	select * from where > order by LIMIT	select * from (select * from where < order by DESC LIMIT) a order by	MySQL SQLite
select TOP * from order by	select TOP * from where > order by	select * from (select TOP * from where < order by DESC) a order by	SQL Server 2005
SET ROWCOUNT select *from order by SET ROWCOUNT 0	SET ROWCOUNT select *from where > order by SET ROWCOUNT 0	SET ROWCOUNT select * from (select * from where < order by DESC) a order by SET ROWCOUNT 0	Sybase, SQL Server 2000
select *from (select * from order by) a where rownum <=	select *from (select * from where > order by) a where rownum <=	select * from (select * from (select * from where < order by DESC) a1 where rownum <=) a2 order by	Oracle 11

Hierarchical query

Some databases provide specialised syntax for hierarchical data.

A window function in is an aggregate function applied to a partition of the result set.

For example,calculates the sum of the populations of all rows having the same city value as the current row.

Partitions are specified using the OVER clause which modifies the aggregate. Syntax:The OVER clause can partition and order the result set. Ordering is used for order-relative functions such as row_number.

Query evaluation ANSI

The processing of a SELECT statement according to ANSI SQL would be the following:^[10]

Window function support by RDBMS vendors

The implementation of window function features by vendors of relational databases and SQL engines differs wildly. Most databases support at least some flavour of window functions. However, when we take a closer look it becomes clear that most vendors only implement a subset of the standard. Let's take the powerful RANGE clause as an example. Only Oracle, DB2, Spark/Hive, and Google Big Query fully implement this feature. More recently, vendors have added new extensions to the standard, e.g. array aggregation functions. These are particularly useful in the context of running SQL against a distributed file system (Hadoop, Spark, Google BigQuery) where we have weaker data co-locality guarantees than on a distributed relational database (MPP). Rather than evenly distributing the data across all nodes, SQL engines running queries against a distributed filesystem can achieve data co-locality guarantees by nesting data and thus avoiding potentially expensive joins involving heavy shuffling across the network. User-defined aggregate functions that can be used in window functions are another extremely powerful feature.

Generating data in T-SQL

Method to generate data based on the union allselect 1 a, 1 b union allselect 1, 2 union allselect 1, 3 union allselect 2, 1 union allselect 5, 1

SQL Server 2008 supports the "row constructor" feature, specified in the standardselect *from (values (1, 1), (1, 2), (1, 3), (2, 1), (5, 1)) as x(a, b)

Sources

• Horizontal & Vertical Partitioning, Microsoft SQL Server 2000 Books Online.

External links

• Windowed Tables and Window function in SQL, Stefan Deßloch
• Oracle SELECT syntax
• Firebird SELECT syntax
• MySQL SELECT syntax
• PostgreSQL SELECT syntax
• SQLite SELECT syntax

Notes and References

1. Web site: Transact-SQL Syntax Conventions . Microsoft . 23 May 2023 .
2. Web site: SQL SELECT Syntax . MySQL.
3. Omitting FROM clause is not standard, but allowed by most major DBMSes.
4. Book: Transact-SQL Reference . SQL Server Language Reference . SQL Server 2005 Books Online . Microsoft . 2007-09-15 . 2007-06-17 .
5. Book: SAS 9.4 SQL Procedure User's Guide. 10 July 2013 . SAS Institute. 2013. 248. 9781612905686. 2015-10-21. Although the UNIQUE argument is identical to DISTINCT, it is not an ANSI standard..
6. Book: Leon. Alexis. Alexis Leon. Leon. Mathews. 1999. Eliminating duplicates - SELECT using DISTINCT. SQL: A Complete Reference. New Delhi. Tata McGraw-Hill Education. 2008. 143. 9780074637081. 2015-10-21. [...] the keyword DISTINCT [...] eliminates the duplicates from the result set..
7. https://www.postgresql.org/docs/9.1/static/tutorial-window.html PostgreSQL 9.1.24 Documentation - Chapter 3. Advanced Features
8. Web site: 9.19.10. The TOP SELECT Option . OpenLink Software . docs.openlinksw.com . 1 October 2019 . en-US.
9. Ing. Óscar Bonilla, MBA
10. Inside Microsoft SQL Server 2005: T-SQL Querying by Itzik Ben-Gan, Lubor Kollar, and Dejan Sarka