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Query by Example (QBE) is a database query language for relational databases. It was devised by Moshé M. Zloof at IBM Research during the mid-1970s, in parallel to the development of SQL . [ 1 ] It is the first graphical query language, using visual tables where the user would enter commands, example elements and conditions.
Git can be used in a variety of different ways, but some conventions are commonly adopted. The command to create a local repo, git init, creates a branch named master. [61] [111] Often it is used as the integration branch for merging changes into. [112] Since the default upstream remote is named origin, [113] the default remote branch is origin ...
[1] [2] [3] Git, the world's most popular version control system, [4] is a distributed version control system. In 2010, software development author Joel Spolsky described distributed version control systems as "possibly the biggest advance in software development technology in the [past] ten years".
SQL was initially developed at IBM by Donald D. Chamberlin and Raymond F. Boyce after learning about the relational model from Edgar F. Codd [12] in the early 1970s. [13] This version, initially called SEQUEL (Structured English Query Language), was designed to manipulate and retrieve data stored in IBM's original quasirelational database management system, System R, which a group at IBM San ...
Versioning symbolic links is considered by some people a feature and some people a security breach (e.g., a symbolic link to /etc/passwd). Symbolic links are only supported on select platforms, depending on the software. Pre-/post-event hooks: indicates the capability to trigger commands before or after an action, such as a commit, takes place.
Relational databases represent all relationships between data in an implicit manner only. [3] [4] For example, the relationships between customers and products (stored in two content-tables and connected with an additional link-table) only come into existence in a query statement (SQL in the case of relational databases) written by a developer.
The relational algebra uses set union, set difference, and Cartesian product from set theory, and adds additional constraints to these operators to create new ones.. For set union and set difference, the two relations involved must be union-compatible—that is, the two relations must have the same set of attributes.
Major DBMSs, including SQLite, [5] MySQL, [6] Oracle, [7] IBM Db2, [8] Microsoft SQL Server [9] and PostgreSQL [10] support prepared statements. Prepared statements are normally executed through a non-SQL binary protocol for efficiency and protection from SQL injection, but with some DBMSs such as MySQL prepared statements are also available using a SQL syntax for debugging purposes.