sql select语句完整的执行顺序（SQL complete select statement execution order）.docVIP

sql select语句完整的执行顺序（SQL complete select statement execution order） SQL complete Select statement execution order SQL complete Select statement execution order: 1, the from clause to assemble data from different data sources; 2, the where clause based on the specified conditions on record for screening; 3, the group by clause divides the data into a plurality of packets; 4, the use of aggregation function for calculation; 5, the use of the having clause screening group; The following conclusion is through the Sql Server2000 on the conclusion of experiments, also can look at the estimated execution plan (Ctrl+L) to check the order of execution. 1. the first implementation of equal sign (=) on the left is a variable expression (a class), then perform equal sign (=) on the left column name expression (class B). For example: update tablename set columnName=@variable @variable=@variable+1, the first implementation of @variable=@variable+1, and then execute columnName=@variable 2. if there is more than one class (or B) expression, according to the execution of a order from left to right (or B). For example: update tablename set columnName=@variable, @variable=@variable+1 @variable=2*@variable, the first implementation of @variable=@variable+1, and then the implementation of @variable=2*@variable, the final implementation of columnName=@variable The 3. column represents the value of the original value is always. For example: update tablename set columnName=colunName+1, columnNameBa group by The select statement can be used in the group by clause will be divided into smaller groups, then the use of Poly Group function returns summary information, each group in addition, you can use the having clause to restrict the returned result set. The group by clause to the query results and returns for the group, Oracle group expression in accordance with the summary information specified in the by clause of the value of the query results. In the query group statement with a by clause,