Structured Query Language (SQL) is a language for querying databases. Questions should include code examples, table structure, sample data, and a tag for the DBMS implementation (e.g. MySQL, PostgreSQL, Oracle, MS SQL Server, IBM DB2, etc.) being used. If your question relates solely to a specific DBMS (uses specific extensions/features), use that DBMS's tag instead. Answers to questions tagged with SQL should use ISO/IEC standard SQL.
How to write a good SQL question?
There are five ingredients to this recipe:
- Provide a clear textual explanation of the problem at hand.
- Provide proper sample data, as DDL (create table statement(s)) and DML (insert statement(s)). The sample data should be enough to demonstrate the problem, including edge cases. Usually, a few relevant rows are enough.
- Provide the expected output for the sample data you've posted.
- Show your attempts to solve the problem.
- Tag properly. In addition to sql, also provide the relevant rdbms tag (i.e. sql-server, oracle, postgresql), and the lowest version you need the solution for (i.e sql-server-2012, oracle10g).
Remember - different products use different dialects of SQL, and this can have a drastic effect on the answers you might get.
Different versions of the same product have different built-in functions and capabilities that might also have a drastic effect on the answers.
Do not include images of data or code!
A link to an online SQL test environment such as SQL Fiddle, Rextester, or db<>fiddle might help, but it is not a replacement for having all the data inside the question.
For more information, Read Why should I provide an MCVE for what seems to me to be a very simple SQL query? and Help me write this query in SQL.
General Information
From Wikipedia:
SQL stands for Structured Query Language (informally) and is usually pronounced as Sequel.
SQL is based on relational algebra. In relational algebra, the word relation is synonymous with the word table. SQL is a standard to use relational algebra in a technical environment.
One subset of the SQL standard is DDL (Data Definition Language), used to create tables and constraints. These include:
CREATEDROPALTER
Another subset is DML (Data Manipulation Language), which is used to modify and view data within the database:
SELECTINSERTUPDATEDELETE
The final "standard" subset of commands is DCL (Data Control Language):
GRANTREVOKE
Many database implementations require the use of SQL, and over the years, vendors have implemented dialects of SQL to provide more functionality and simplify it. Because of these deviations from the standard, SQL is fractured – syntax that works on one implementation does not necessarily work on another.
ISO/IEC (formerly ANSI) standards have been beneficial in resolving such situations, but adoption is selective. Queries conforming to these standards should be portable to other databases, though performance may vary.
Most DBMSs have additional languages for writing stored procedures. In Oracle, it is PL/SQL (Procedural Language/Structured Query Language). In PostgreSQL, it's PL/pgSQL (Procedural Language/PostgreSQL). Outside of stored procedures or functions, Oracle and PostgreSQL use SQL. Thus the tags plsql and plpgsql should only be used for problems directly related to writing stored procedures. Microsoft SQL Server uses the term T-SQL (Transact-SQL)(tsql) for both "plain" SQL (queries, DML, ..) and the language used for stored procedures.
List of Procedural Extensions
- ANSI/ISO Standard: SQL/PSM (SQL / Persistent Stored Modules)
- Interbase/Firebird: PSQL (Procedural SQL)
- IBM DB2 SQL: PL/SQL (Procedural Language, implements SQL/PSM)
- IBM Informix: SPL (Stored Procedural Language)
- IBM Netezza: NZPLSQL (based on Postgres PL/pgSQL)
- Microsoft/Sybase: T-SQL (Transact-SQL)
- Mimer SQL: SQL/PSM (SQL / Persistent Stored Module, implements SQL/PSM)
- MySQL: SQL/PSM (SQL / Persistent Stored Module, implements SQL/PSM)
- MonetDB: SQL/PSM (SQL / Persistent Stored Module, implements SQL/PSM)
- NuoDB: SSP (Starkey Stored Procedures)
- Oracle: PL/SQL (Procedural Language / SQL, based on Ada)
- PostgreSQL: PL/pgSQL (Procedural Language / PostgreSQL Structured Query Language, implements SQL/PSM)
- Sybase: Watcom-SQL (SQL Anywhere Watcom-SQL Dialect)
- Teradata: SPL (Stored Procedural Language)
- SAP: SAP HANA (SQL Script)
Tagging Recommendation
This tag should be used for general SQL programming language questions, in addition to tags for specific products. For example, questions about Microsoft SQL Server should use the sql-server tag, while questions regarding MySQL should use the mysql tag. SQL is the umbrella under which these products exist; tagging them by product (including version, e.g oracle11g, sql-server-2008, hana) is the easiest way to know what functionality is available for the task at hand. It is very common for mysql questions to omit this tag because query discussions on MySQL are more often stated as MySQL rather than SQL in general.
Please read this summary about the SQL standard (The 1992 one in this case, broadly implemented) and if you can, refer to the book itself.
More specific tags
When you are asking a question about SQL you can also add more specific tags. Here is the list of available tags:
- sqlbulkcopy
- sqlconnection
- sqlcommand
- sql-copy
- sql-convert
- sql-delete
- sql-drop
- sqldatetime
- sql-date-functions
- sql-function
- sql-job
- sql-like
- sql-limit
- sql-merge
- sql-pivot
- sql-order-by
- sql-returning
- sql-server-job
- to-date
- triggers
- sql-timestamp
- sql-update
- sql-view
- stored-procedures
- having
- where-clause
- count
- group-by
- greatest-n-per-group
- join
- left-join
- inner-join
- outer-join
- self-join
- cross-join
- right-join
- full-outer-join
- natural-join
- equi-join
- non-equi-join
Implementation specific tags
You can specify your question by adding the implementation you used as a tag.
- amazon-athena
- apache-spark-sql
- db2
- google-query-language
- hana
- hiveql
- impala
- ms-access
- mimer-sql
- mysql
- oracle
- postgresql
- presto
- snowflake-cloud-data-platform
- sqlite
- sql-server
- sybase
- teradata
- trino
Other Resources
Free SQL Programming Books
- Developing Time-Oriented Database Applications in SQL [PDF]
- Use The Index, Luke!: A Guide To SQL Database Performance
- SQL Tutorial For Starters
- SQL - Free books 2
Free SQL/Database Online Courses
SQL/Database Online Tutorial
SQL reserved words overview
Online Testing
While you should always provide complete code examples (e.g., schema, data sample and expected result) in your question or answer, you can also isolate problematic code and reproduce it in an online environment:
- SQL Fiddle MySQL 5.6, Oracle 11g R2, PostgreSQL 9.6, PostgreSQL 9.3, SQLite (WebSQL), SQLite (SQL.js), SQL Server 2017
- Rextester: SQL Server, PostgreSQL, MySql, Oracle
- Stack Exchange Data Explorer Microsoft SQL Server 2016
- db-fiddle MySQL 5.5, 5.6, 5.7, 8.0 PostgreSQL 9.4, 9.5, 9.6, 10, 11, 12, 13 SQLite 3.26, 3.30
- db <> fiddle Db2 Developer-C 11.1, Firebird 3.0, MariaDB 10.3, 10.4, 10.5, 10.6, MySQL 5.5, 5.6, 5.7, 8.0, Oracle 11g Release 2, Postgres 9.5, 9.6, 10, 11, 12,13, SQLite 3.27, SQL Server 2014, 2016 and 2017, 2017 (Linux), 2019, 2019 (Linux)
- Hue demo Hive, Trino, MySQL, Flink SQL, Phoenix SQL, ksqlDB, Dask SQL, SparkSQL
