In the late 1960s/early 1970s, specialised data management software appeared – the first database management systems (DBMS). These early DBMS were either hierarchical (tree) or network (CODASYL) databases which were complex and inflexible and could not easily be modified for new applications or reorganising the data.
In 1974 IBM started a project called System/R to prove the theory of relational databases. This led to the development of a query language called SEQUEL (Structured English Query Language) later renamed to Structured Query Language (SQL) for legal reasons and now the query language of all databases.
In 1978 a prototype System/R implementation was evaluated at a number of IBM customer sites.
By 1979 the project finished with the conclusion that relational databases were a feasible commercial product.
1) SYS (essential core database structures and utilities)
2) SYSTEM (additional core database structures and utilities, and privileged account)
3) OUTLN (utilized to store metadata for stored outlines for stable query-optimizer execution plans.)
4) BI, IX, HR, OE, PM, and SH (expanded sample schemas containing more data and structures than the older SCOTT schema).
The following statement selects rows from the employees table with the department number of 30:
WHERE department_id = 30;
The following statement selects the name, job, salary and department number of all employees except purchasing clerks from department number 30:
SELECT last_name, job_id, salary, department_id
WHERE NOT (job_id = 'PU_CLERK' AND department_id = 30);
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