Purpose
This function is an analytic function that retrieves the data of the preceding offset rows of the same field in a query and stores it as an independent column in the table. This operation can replace a self-join on the table.
Syntax
LAG { (value_expr [,offset [,default]]) [{ RESPECT|IGNORE } NULLS ]
| (value_expr [{ RESPECT|IGNORE } NULLS ] [,offset [,default] ])
}
OVER([query_partition_clause] order_by_clause)
Parameters
Parameter |
Description |
|---|---|
| value_expr | Specifies the field to be queried. Notice You cannot nest value_expr in the LAG function or other analytic functions. |
| offset | Specifies the offset for querying value_expr. It is an integer greater than 0 and is optional. The default value is 1. |
| default | Specifies the default value. If the offset exceeds the window range, the value of default is returned. It is optional. The default value is NULL. |
| {RESPECT | IGNORE} NULLS | Specifies whether to ignore NULL values. It is optional. The default value is RESPECT NULLS.
|
| OVER | Defines the window for calculation using the OVER clause. For more information, see Analytic Function Description. |
Return type
The data type of the return value is not limited.
Examples
An emp_msg table has been created. Query the emp_msg table, replace the first three values with XXX, and append values sorted in ascending order by the ename field starting from the fourth value.
obclient> SELECT * FROM emp_msg;
+--------+--------+------+------+
| DEPTNO | ENAME | SAL | MGR |
+--------+--------+------+------+
| 10 | CLARK | 2750 | 7839 |
| 10 | KING | 5300 | NULL |
| 10 | MILLER | 1600 | 7782 |
| 20 | ADAMS | 1400 | 7788 |
| 20 | FORD | 3300 | 7566 |
| 20 | JONES | 3275 | 7839 |
| 20 | SCOTT | 3300 | 7566 |
| 20 | SMITH | 1100 | 7902 |
| 30 | ALLEN | 1900 | 7698 |
| 30 | BLAKE | 3150 | 7839 |
| 30 | JAMES | 1250 | 7698 |
| 30 | MARTIN | 1550 | 7698 |
| 30 | TURNER | 1800 | 7698 |
| 30 | WARD | 1550 | 7698 |
| 30 | SCLARK | 1750 | 7839 |
+--------+--------+------+------+
15 rows in set
obclient> SELECT deptno,ename, sal,LAG(ename,3,'XXX') OVER (ORDER BY ename) AS new_ename
FROM emp_msg;
+--------+--------+------+-----------+
| DEPTNO | ENAME | SAL | NEW_ENAME |
+--------+--------+------+-----------+
| 20 | ADAMS | 1400 | XXX |
| 30 | ALLEN | 1900 | XXX |
| 30 | BLAKE | 3150 | XXX |
| 10 | CLARK | 2750 | ADAMS |
| 20 | FORD | 3300 | ALLEN |
| 30 | JAMES | 1250 | BLAKE |
| 20 | JONES | 3275 | CLARK |
| 10 | KING | 5300 | FORD |
| 30 | MARTIN | 1550 | JAMES |
| 10 | MILLER | 1600 | JONES |
| 30 | SCLARK | 1750 | KING |
| 20 | SCOTT | 3300 | MARTIN |
| 20 | SMITH | 1100 | MILLER |
| 30 | TURNER | 1800 | SCLARK |
| 30 | WARD | 1550 | SCOTT |
+--------+--------+------+-----------+
15 rows in set