Editing SQL (section)

===Overview===
SQL implementations are incompatible between vendors and do not necessarily completely follow standards. In particular, date and time syntax, string concatenation, <code>NULL</code>s, and comparison [[case sensitivity]] vary from vendor to vendor. [[PostgreSQL]]<ref name="About PostgreSQL" /> and [[Mimer SQL]]<ref name="Mimer SQL, Built on Standards" /> strive for standards compliance, though PostgreSQL does not adhere to the standard in all cases. For example, the folding of unquoted names to lower case in PostgreSQL is incompatible with the SQL standard,<ref>{{cite web|url=https://www.postgresql.org/docs/current/static/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS|title=4.1. Lexical Structure|date=2018|website=PostgreSQL documentation|access-date=11 December 2018|archive-date=5 October 2018|archive-url=https://web.archive.org/web/20181005190818/https://www.postgresql.org/docs/current/static/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS|url-status=live}}</ref> which says that unquoted names should be folded to upper case.<ref>{{cite web|url=http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt|title=(Second Informal Review Draft) ISO/IEC 9075:1992, Database Language SQL, Section 5.2, syntax rule 11|date=30 July 1992|access-date=8 October 2009|archive-date=21 June 2006|archive-url=https://web.archive.org/web/20060621035823/http://www.contrib.andrew.cmu.edu/%7Eshadow/sql/sql1992.txt|url-status=live}}</ref> Thus, according to the standard, <code>Foo</code> should be equivalent to <code>FOO</code>, not <code>foo</code>.

Popular implementations of SQL commonly omit support for basic features of Standard SQL, such as the <code>DATE</code> or <code>TIME</code> data types.  The most obvious such examples, and incidentally the most popular commercial and proprietary SQL DBMSs, are Oracle (whose <code>DATE</code> behaves as <code>DATETIME</code>,<ref name="Ora DATE" /><ref name="Ora DATETIME" /> and lacks a <code>TIME</code> type)<ref name="Ora TIME" /> and MS SQL Server (before the 2008 version). As a result, SQL code can rarely be ported between database systems without modifications.