CSE 278 Lecture 13: SQL 3
18 Oct 2018
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Document Summary
Often we search in strings for a particular pattern. The construct s like pat, where pat stands for pattern, strings that contains in addition to 2 characters (metasymbols). % standing for 0 or more characters. Since they are special format, variable length characters, a keyword is included in the constant: Other delimiters are formats are also recognized. Timestamp is similar, but it is automatically updated. Attribute with value of null may be interpreted in following ways. Unknown: it"s value that exists but will be undetermined at that time. Inapplicable/does not apply: often abbreviated in forma n/a. Withheld: for some reason like a password or social security. It"s important to consider all 3 possibilities to avoid pitfalls. When doing this simple select, the resulting relation r" is formed by the true tuples for predicate c. There are some complicated rules on this, included what is called three-valued logic. Also, don"t check for null with the natural dob = null.
