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SQL is a language intended for table transformation. The input is one or more tables, the output is also a table and every instruction applied in the middle gets and outputs a table. Everything in |
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SELECT clause is about tables. |
Tables have columns and columns have a name and a data type: INT, STRING, DATE, BOOLEAN, etc.
Table rows are data records and the intersection with a column is a cell value (which can be NULL).
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The execution order of a SQL query is not the same as the used to write it:
FROM → JOIN ON → WHERE → SELECT → DISCTINCT → GROUP BY → HAVING → ORDER BY → OFFSET → LIMIT
Everything starts with a FROM table.
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The FROM table (1) is JOIN with other tables ON some constraints to produce a new table.
Then the table (
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2) above is pruned horizontally with WHERE by dropping all the records
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not matching the conditions
The new table (
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3) is pruned vertically then, to match the SELECT columns only.
DISTINCT removes all the duplicate records in (
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4)
GROUP BY will create a new table from (
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5) grouping by the
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provided columns.
HAVING will prune the previous (
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6) table by removing all the records
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that don't match the conditions. HAVE acts similarly to the WHERE clause for the GROUP BY.
The filtered grouped table (
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7) will be ORDER BY some columns
OFFSET N will skip N first records in (
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8)
and LIMIT M will take the output table from (
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9) and remove all the records after the M first ones.
The result of all the above operations (many optional) will output another temporary table that will be destroyed once it has been read.
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