SQL step on pit notes

1. Sql execution order of: From -> Where -> Group By -> Having -> Select -> Order By
2. Group By clause can not use the Select clause alias (because of the execution order)
3. Select clause, Having clause, Order By clause can use Count (*) like aggregate functions
4. Where clause is used to specify the data line condition, Having clause specifies grouping criteria
5. Having clause components: constant, aggregate functions, Group By column name specified in clause (i.e., polymerization key)
6. In the polymerization using the functions: the Select clause have strict requirements: constants specified, aggregate functions, Group By clause column name (i.e., polymerization key)
7. Ways to create a view: CREAT VIEW view name (<1 view column name>, <View column name 2>, ......) AS <Select statement>
8. Order By clause can not use the view is defined
9. updating. . . .

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