What is the order of sql execution?

The execution sequence of SQL statements

MySQL statements are divided into 11 steps in total, as marked in the figure below, first The FROM operation is always executed, and the LIMIT operation is executed last. Each operation will generate a virtual table, which is used as an input for processing. However, these virtual tables are transparent to the user, but only the last virtual table will be returned as a result. If a clause is not specified in the statement, the corresponding step will be skipped.

Let’s analyze each stage of query processing in detail

FORM: For the table on the left and the table on the right of FROM Table calculates Cartesian product. Generate virtual table VT1

ON: Perform ON filtering on virtual table VT1, and only those rows that meet the will be recorded in virtual table VT2.

JOIN: If OUTER JOIN (such as left join, right join) is specified, the unmatched rows in the retained table will be added to the virtual table VT2 as external rows, resulting in a virtual table VT3, rug from sub- If the sentence contains more than two tables, then steps 1 to 3 will be repeated for the result VT3 generated by the previous join connection and the next table until all tables are processed

WHERE: Perform WHERE condition filtering on virtual table VT3. Only records that meet the will be inserted into virtual table VT4.

GROUP BY: Group the records in VT4 according to the columns in the group by clause to generate VT5.

CUBE | ROLLUP: Perform cube or rollup operations on table VT5 to generate Table VT6.

HAVING: Apply having filter to virtual table VT6. Only records that meet the will be inserted into virtual table VT7.

SELECT: Perform a select operation, select the specified column, and insert it into virtual table VT8.

DISTINCT: Deduplicate records in VT8. Generate virtual table VT9.

ORDER BY: Sort the records in virtual table VT9 according to to generate virtual table VT10.

LIMIT: Take out the records of the specified row and generate Virtual table VT11, and returns the result.

Writing order: select... from... where.... group by... having... order by... limit [offset,]

(rows)

Execution order: from... where...group by... having.... select... order by... limit

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