Questions tagged [sql]

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:

  1. Provide a clear textual explanation of the problem at hand.
  2. 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.
  3. Provide the expected output for the sample data you've posted.
  4. Show your attempts to solve the problem.
  5. Tag properly. In addition to , also provide the relevant rdbms tag (i.e. , , ), and the lowest version you need the solution for (i.e , ).
    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:

  • DROP

Another subset is DML (Data Manipulation Language), which is used to modify and view data within the database:


The final "standard" subset of commands is DCL (Data Control Language):


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 and should only be used for problems directly related to writing stored procedures. Microsoft SQL Server uses the term T-SQL (Transact-SQL)() 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 tag, while questions regarding MySQL should use the tag. SQL is the umbrella under which these products exist; tagging them by product (including version, e.g , , ) is the easiest way to know what functionality is available for the task at hand. It is very common for 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:

Implementation specific tags

You can specify your question by adding the implementation you used as a tag.

Other Resources

Free SQL Programming Books

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
665031 questions
28 answers

What is the difference between "INNER JOIN" and "OUTER JOIN"?

Chris de Vries
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How do I UPDATE from a SELECT in SQL Server?

In SQL Server, it is possible to insert rows into a table with an INSERT.. SELECT statement: INSERT INTO Table (col1, col2, col3) SELECT col1, col2, col3 FROM other_table WHERE sql = 'cool' Is it also possible to update a table with SELECT? I…
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44 answers

Add a column with a default value to an existing table in SQL Server

How can I add a column with a default value to an existing table in SQL Server 2000 / SQL Server 2005?
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7 answers

How does database indexing work?

Given that indexing is so important as your data set increases in size, can someone explain how indexing works at a database-agnostic level? For information on queries to index a field, check out How do I index a database column.
Xenph Yan
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27 answers

How can I prevent SQL injection in PHP?

If user input is inserted without modification into an SQL query, then the application becomes vulnerable to SQL injection, like in the following example: $unsafe_variable = $_POST['user_input']; mysql_query("INSERT INTO `table` (`column`) VALUES…
Andrew G. Johnson
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39 answers

Finding duplicate values in a SQL table

It's easy to find duplicates with one field: SELECT email, COUNT(email) FROM users GROUP BY email HAVING COUNT(email) > 1 So if we have a table ID NAME EMAIL 1 John 2 Sam 3 Tom 4 Bob …
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47 answers

How to concatenate text from multiple rows into a single text string in SQL Server

Consider a database table holding names, with three rows: Peter Paul Mary Is there an easy way to turn this into a single string of Peter, Paul, Mary?
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46 answers

How to return only the Date from a SQL Server DateTime datatype

SELECT GETDATE() Returns: 2008-09-22 15:24:13.790 I want that date part without the time part: 2008-09-22 00:00:00.000 How can I get that?
Eddie Groves
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2 answers

What's the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL JOIN?

What's the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL JOIN in MySQL?
Lion King
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4 answers

Inserting multiple rows in a single SQL query?

I have multiple set of data to insert at once, say 4 rows. My table has three columns: Person, Id and Office. INSERT INTO MyTable VALUES ("John", 123, "Lloyds Office"); INSERT INTO MyTable VALUES ("Jane", 124, "Lloyds Office"); INSERT INTO MyTable…
20 answers

Select first row in each GROUP BY group?

I'd like to select the first row of each set of rows grouped with a GROUP BY. Specifically, if I've got a purchases table that looks like this: SELECT * FROM purchases; My…
David Wolever
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46 answers

How to query MongoDB with "like"

I want to query something with SQL's like query: SELECT * FROM users WHERE name LIKE '%m%' How can I achieve the same in MongoDB? I can't find an operator for like in the documentation.
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34 answers

Insert results of a stored procedure into a temporary table

How do I do a SELECT * INTO [temp table] FROM [stored procedure]? Not FROM [Table] and without defining [temp table]? Select all data from BusinessLine into tmpBusLine works fine. select * into tmpBusLine from BusinessLine I am trying the same, but…
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27 answers

Insert into ... values ( SELECT ... FROM ... )

I am trying to INSERT INTO a table using the input from another table. Although this is entirely feasible for many database engines, I always seem to struggle to remember the correct syntax for the SQL engine of the day (MySQL, Oracle, SQL Server,…
Claude Houle
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41 answers

Table Naming Dilemma: Singular vs. Plural Names

Academia has it that table names should be the singular of the entity that they store attributes of. I dislike any T-SQL that requires square brackets around names, but I have renamed a Users table to the singular, forever sentencing those using…
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