Constraints: Constraints are rules which are imposed on columns or table. Constraints allows only data that meets requirement to be entered. It can be applied on both column and table.
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The first version of SQL was developed by IBM called Sequel as part of the System R project. SQL stands for structured query language and is a combination of simple english words. SQL is used as a language to communicate with database. Same SQL code can run on multiple database albeit some database have slightly different syntax than others but still they have to maintain specification of SQL.
ER or Entity Reletionship model is used to create and model the data which is simple, non technical and easily understood by people describing a diagram with relation. It helps us to analyze requirements systematically to produce a well designed database.