Why does wechat use SQLite to save chat records?

SQLite It is a database underestimated by everyone , But some people think it is a toy database that is not suitable for production environment . in fact ,SQLite Is a very reliable database , It can handle TB Level of data , But it has no network layer . Next , This article will discuss with you SQLite The latest in the past year SQL function .

SQLite “ It's just ” A library , It is not a traditional server . therefore , In some cases , It's really inappropriate . however , On quite a few other occasions , It is the most suitable choice .SQLite It claims to be the most widely deployed and used database engine . I think it's very possible , because SQLite There are no copyright restrictions . No matter when , As long as developers want to use SQL Store structured data in files ,SQLite Should be the preferred solution .

SQLite Of SQL Dialects are also very powerful . It is better than MySQL Four years ago, I began to support with sentence . lately , It also implements support for window functions , It's just better than MySQL Five months late .

Next , This article will introduce SQLite stay 2018 Newly added in SQL function , That is to say SQLite From version 3.22.0 To 3.26.0 Newly added SQL function .

Specific contents include ：

1. Boolean literals and judgments

2. Window function

3. Filter Clause

4. Insert … on conflict (“Upsert”)

5. To be ranked high

6. stay Modern-SQL.com Go up next

Boolean variables and judgments

SQLite Support “ false ” Boolean value ： It accepts Boolean As the name of the type , But it treats it as an integer （ This is very similar to MySQL）. Truth value true and false By the numerical value 1 and 0 Express （ This and C The language is the same ）.

From version 3.23.0 Start ,SQLite Put keywords true and false Separate numbers 1 and 0 Express , And support is [not] true | false Judgment statement of . Now? , It no longer supports keywords unknown. Developers can use null values null Instead of , because unknown and null The Boolean value of is the same .

stay INSERT and UPDATE In the sentence , Literal true and false Can be greatly improved values and set Readability of clauses .

is [not] true | false This judgment statement is very useful , It is different from the meaning of comparison ：

Let's compare

WHERE c <> FALSE

and

WHERE c IS NOT FALSE

In the example above , If c yes null, that c <> false The result is unknown.

This is because WHERE Clause only accepts the result as true Value , It will filter out the result as false or unknown Value . such , It will remove the corresponding row from the result .

Corresponding to this , If c yes null, that ,c is not false It turns out that true. therefore , the second WHERE The clause will also contain c yes null The line of .

To achieve the same effect , Another method you can use is to add separate processing null Value clause . That is, use statements :

WHERE c <> FALSE
   OR c IS NULL

This form of statement is longer and has some redundant statements （c Used twice ）. Cut a long story short , have access to is not false Judgment replaces this or…is-null The sentence of . More details , Please refer to “Binary Decisions Based on Three-Valued Results”.

SQLite Support for Boolean literals and Boolean judgments in is now close to other open source databases , The only difference is SQLite I won't support it is[not] unknown（ You can use is [not] null Instead of ）. Interestingly , These functions are not yet available in the commercial products mentioned below .

picture

0： Only support true,false. I won't support it notknown, if necessary , use null Instead of

1： I won't support it is [not] unknown, if necessary , use is [not] null Instead of

Window function

SQLite 3.25.0 Introduced window function . If you know the window function , Then I know this is a big thing . If you don't understand window functions , Please learn how to use by yourself . This article will not specifically explain window functions , But please believe that ： It's the most important “ modern ”SQL characteristic .

SQLite Yes over Clause support is very close to other databases . The only notable limitation is range Statement does not support numbers or spacing （ Support only current row and unbounded preceding|following）. In the release sqlite 3.25.0 when ,SQL Server and PostgreSQL With the same restrictions .PostgreSQL 11 Eliminate this limitation .

picture

0： There is no change

1：Range Range definitions do not support datetime type

2：Range The scope does not accept keywords ( Only support unbounded and current row)

SQLite The support for window functions is leading in the industry . Functions that it does not support are also not supported in some other major products （ In an aggregate statement distinct,width_bucket, respect|ignore nulls and from first|last Such statements ）.

0： Again, there is no ORDER BY sentence

1： Negative offset is not allowed ,nulls Specific processing of ：lead(, 'IGNORE NULLS'), Here is the string parameter

2： There is no default value ( The third parameter ), I won't support it respect|ignore nulls sentence

3： Negative offset is not allowed , I won't support it ignore nulls sentence

4： Negative offset is not allowed

5： I won't support it respect|ignore nulls sentence

6： Negative offset is not allowed , I won't support it respect|ignore nulls sentence

7：nulls Specific processing of ：first_value(, 1, null, 'IGNORE NULLS') , Here is the string parameter .

8： I won't support it ignore nulls sentence

9： I won't support it ignore nulls Statement and from last sentence

Filter statements

although filter Statements are just grammatical sugar —— You can also easily use expressions to get the same result —— I think it is also an essential grammar sugar , Because it makes it easier for people to learn and understand SQL sentence .

Look at the following select Clause , Which one do you think is easier to understand ？

SELECT SUM(revenue) total_revenue
     , SUM(CASE WHEN product = 1 
                THEN revenue
            END
          ) prod1_revenue
   ...

and

SELECT SUM(revenue) total_revenue
     , SUM(revenue) FILTER(WHERE product = 1) prod1_revenue
   ...

This example summarizes filter The function of clause ： It is the suffix of aggregate function , It can be based on specific conditions before aggregation , Filter out the corresponding lines .pivot The technology is filter Clause is the most common use case . This includes adding entity attribute values （EAV） Attributes in the model are converted into columns of the table , If you want to know more , You can refer to the link “filter-Selective Aggregates”（https://modern-sql.com/feature/filter）.

SQLite From version 3.25.0 Start , In the use of over Clause supports filter Clause , But in use group by Clause is not yet supported in aggregate functions . Unfortunately , This means that you still cannot SQLite Use in filter Statement to handle the above situation . You must use it as before case expression . I really hope SQLite We can do this as soon as possible .

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Insert … on conflict (“Upsert”)

SQLite From version 3.24.0 Start , Introduced “upsert” Concept ： It's a insert sentence , You can handle the conflict between primary key and unique constraint gracefully . You can choose to ignore these conflicts （ stay on conflict Do nothing in the sentence ） Or update the current line （ stay on conflict Statement ）.

This is a unique SQL Expand , That is, it is not a standard SQL Part of , Therefore, it is gray in the following matrix . however ,SQLite Compliance and PostgreSQL The same syntax to achieve this function 0. This standard provides a reference to merge Statement support .

And PostgreSQL Different ,SQLite There is a problem in the following statement .

INSERT INTO target
SELECT *
  FROM source
    ON CONFLICT (id)
    DO UPDATE SET val = excluded.val

According to the documentation , This is because the parser cannot judge keywords ON yes SELECT The connection constraint of the statement is still upsert The beginning of a clause . You can solve this problem by adding clauses to the query , for example where true.

INSERT INTO target
SELECT *
  FROM source
 WHERE true
    ON CONFLICT (id)
    DO UPDATE SET val = excluded.val

picture

0： Also record insert、update、delete and merge Error message of operation (“DML error logging”)

1：On conflict The statement cannot be next to the query from sentence , if necessary , You can add  where true Statement to separate .

To be ranked high

SQLite Another unique feature introduced is renaming columns in the benchmark database table 1. The standard SQL This kind of function is not supported 2.

SQLite Follow the syntax commonly used by other products to rename the list ：

ALTER TABLE … RENAME COLUMN … TO

picture

0： Please refer to sp_rename.

Other messages

stay 2018 year ,SQLite In addition to the SQL Grammatical changes , There are also some application program interfaces （API） The change of . You can refer to sqlite.com（https://www.sqlite.org/news.html） See the news section on the for more detailed information .

Footmark :

• 0：SQLite Generally follow PostgreSQL grammar ,Richard Hipp Call this PostgreSQL What will be done （WWPD）.

• 1： The benchmark database table refers to Create table Statement to create a database table . Derived database tables （ Such as Select Statement returns the query result set ） The column names in can be passed through SELECT sentence 、FROM Sentence or WITH Statement to change

• 2： as far as I am concerned , Perhaps this function can be simulated by updatable views or derived columns .

original text ：https://modern-sql.com/blog/2019-01/sqlite-in-2018

Author's brief introduction ：Markus Winand Provide efficient SQL train . His published works 《SQL Performance Explained》 It has become a standard reading for developers .

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