Recent Movable case Project use ActiveReports Report designer When designing a report template , There is a problem of multi-level classification : It is necessary to summarize the sales amount of a certain department and its subordinate departments , Because the hierarchy of subordinates is uncertain , So the way of splicing sub query obviously can not meet the requirements , After some experiments , Take advantage of CTE(Common Table Expression) It's easy to solve this problem !

give an example : There is a list of departments as follows

And employee list

If you want to check all the employees in the Northwest ( Including northwest 、 Xi'an 、 lanzhou ), As shown in the figure below :

How to use CTE How to achieve it ?

Talk is cheap. Show me the code

-- The following code USES SQLite 3.18.0 The test passed 
[depts]([dept_id]) AS(
SELECT [d].[dept_id]
FROM [dept] [d]
JOIN [employees] [e] ON [d].[dept_id] = [e].[dept_id]
WHERE [e].[emp_name] = ' The northwest - The manager '
SELECT [d].[dept_id]
FROM [dept] [d]
JOIN [depts] [s] ON [d].[parent_id] = [s].[dept_id]
FROM [employees]
WHERE [dept_id] IN (SELECT [dept_id]
FROM [depts]);

Maybe some students are right CTE(Common Table Expression) Not familiar with , Let's talk about it briefly , Interested students can google Or Baidu , Introduce a lot ( Here we use SQLite give an example ):

I prefer to weigh CTE(Common Table Expression) by “ Common table variables ” instead of “ Common expression ”, Because in terms of behavior and usage scenarios ,CTE More often than not, it's the production of ( Iterate or not ) Result set , For subsequent statements ( Inquire about 、 Insert 、 Delete or update ), For example, the above example is a typical example of iteratively traversing tree structured data .

CTE The advantages of :

  • Recursion makes it necessary to use temporary tables 、 Logic that can only be completed by stored procedures , adopt SQL You can do it , Especially for the data model of some trees or graphs
  • Because it's a temporary result set in the session , There is no need to display the statement or destroy
  • After the rewrite SQL Sentence readability is improved ( Only when you can see clearly can you revise it )
  • Give the database engine the possibility to optimize the execution plan ( This is not for sure , It needs to be specific CTE The realization of ), Optimized execution plan , Naturally, performance can go up

For better explanation CTE The ability of , Here are two examples ( from SQLite Official document )

Mandelbrot assembly (Mandelbrot set)

-- The following code USES SQLite 3.18.0 The test passed 
xaxis(x) AS (VALUES(-2.0) UNION ALL SELECT x+0.05 FROM xaxis WHERE x<1.2),
yaxis(y) AS (VALUES(-1.0) UNION ALL SELECT y+0.1 FROM yaxis WHERE y<1.0),
m(iter, cx, cy, x, y) AS (
SELECT 0, x, y, 0.0, 0.0 FROM xaxis, yaxis
SELECT iter+1, cx, cy, x*x-y*y + cx, 2.0*x*y + cy FROM m
WHERE (x*x + y*y) < 4.0 AND iter<28
m2(iter, cx, cy) AS (
SELECT max(iter), cx, cy FROM m GROUP BY cx, cy
a(t) AS (
SELECT group_concat( substr(' .+*#', 1+min(iter/7,4), 1), '')
SELECT group_concat(rtrim(t),x'0a') FROM a;

Results after operation , Here's the picture :( Use SQLite Expert Personal 4.2 x64)

Sudoku question (Sudoku)

Suppose there is a problem similar to the figure below :

-- The following code USES SQLite 3.18.0 The test passed 
input(sud) AS (
digits(z, lp) AS (
VALUES('', 1)
CAST(lp+1 AS TEXT), lp+1 FROM digits WHERE lp<9
x(s, ind) AS (
SELECT sud, instr(sud, '.') FROM input
substr(s, 1, ind-1) || z || substr(s, ind+1),
instr( substr(s, 1, ind-1) || z || substr(s, ind+1), '.' )
FROM x, digits AS z
WHERE ind>0
FROM digits AS lp
WHERE z.z = substr(s, ((ind-1)/9)*9 + lp, 1)
OR z.z = substr(s, ((ind-1)%9) + (lp-1)*9 + 1, 1)
OR z.z = substr(s, (((ind-1)/3) % 3) * 3
+ ((ind-1)/27) * 27 + lp
+ ((lp-1) / 3) * 6, 1)

Execution results ( The number in the result is the answer in the corresponding grid )

attach :SQLite in CTE(WITH keyword ) Grammar illustration :





CTE It's a powerful tool to solve some specific problems , But understanding and proper use is the premise , In deciding to put some of the existing SQL Refactor to CTE Before , Make sure you have a clear understanding of the existing sentences and have a clear understanding of CTE Enough learning !Good Luck~~~

The attachment Use of SQL Script

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