国产av日韩一区二区三区精品,成人性爱视频在线观看,国产,欧美,日韩,一区,www.成色av久久成人,2222eeee成人天堂

Home php教程 php手冊 NOT NULL 和NULL

NOT NULL 和NULL

Jun 13, 2016 pm 12:33 PM
NOT null 。 for and designation null

NOT?NULL?|?NULL:指定該列是否允許為空。如果既不指定NULL也不指定NOT?NULL,列被認為指定了NULL
在?MySQL?中,?
為一個?NOT?NULL?字段設(shè)置?NULL?值?,?
它并不會出錯,?
MySQL?會自動將?NULL值轉(zhuǎn)化為該字段的默認值,?
那怕是你在表定義時沒有明確地為該字段設(shè)置默認值?
:》一般來說MySQL會自動為你添加默認值的,?

比如?
為一個?NOT?NULL?的?整型?賦?NULL?值,?
結(jié)果是?0?

CHAR?型?
->?''空值?

DATATIME?
->'0000-00-00?00:00:00'?

等等
Top

2?樓UNICORN_BX(天天天藍)回復(fù)于?2003-01-13?11:22:18?得分?0?也就是說,MySQL中NOT?NULL并不是一個約束條件了.?

如果字段設(shè)置為NULL,則插入記錄時沒有給予該字段的值,那么MySQL自動用Default的值,如果沒有Default,則無論是該字段是什么類型該字段值NULL.?

這是不是就是MySQL中NOT?NULL和NULL所起到的作用,即當不為定義Default值的時候,給予的默認值的不同.

Statement of this Website
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn

Hot AI Tools

Undress AI Tool

Undress AI Tool

Undress images for free

Undresser.AI Undress

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

AI Clothes Remover

Online AI tool for removing clothes from photos.

Clothoff.io

Clothoff.io

AI clothes remover

Video Face Swap

Video Face Swap

Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Tools

Notepad++7.3.1

Notepad++7.3.1

Easy-to-use and free code editor

SublimeText3 Chinese version

SublimeText3 Chinese version

Chinese version, very easy to use

Zend Studio 13.0.1

Zend Studio 13.0.1

Powerful PHP integrated development environment

Dreamweaver CS6

Dreamweaver CS6

Visual web development tools

SublimeText3 Mac version

SublimeText3 Mac version

God-level code editing software (SublimeText3)

How to solve the problem of jQuery AJAX error 403? How to solve the problem of jQuery AJAX error 403? Feb 23, 2024 pm 04:27 PM

How to solve the problem of jQueryAJAX error 403? When developing web applications, jQuery is often used to send asynchronous requests. However, sometimes you may encounter error code 403 when using jQueryAJAX, indicating that access is forbidden by the server. This is usually caused by server-side security settings, but there are ways to work around it. This article will introduce how to solve the problem of jQueryAJAX error 403 and provide specific code examples. 1. to make

ASUS motherboard options compatible with R55600 (including R55600u and 5600h) ASUS motherboard options compatible with R55600 (including R55600u and 5600h) Jan 02, 2024 pm 05:32 PM

Which ASUS motherboard should be paired with the R55600? The ASUS ROGStrixB550-FGaming motherboard is an excellent choice. It is perfectly compatible with Ryzen55600X processor and provides excellent performance and features. This motherboard has a reliable power supply system, can support overclocking, and provides a wealth of expansion slots and ports to meet daily use and gaming needs. ROGStrixB550-FGaming is also equipped with high-quality audio solutions, fast network connections and reliable heat dissipation design to ensure that the system remains efficient and stable. In addition, this motherboard adopts a gorgeous ROG style and is equipped with gorgeous RGB lighting effects, adding visual enjoyment to your computer. All in all, ASUS ROGStri

Which one is better, Celeron g4900 or i36100? (Which one is better, Celeron g4900 or i34170?) Which one is better, Celeron g4900 or i36100? (Which one is better, Celeron g4900 or i34170?) Jan 01, 2024 pm 06:01 PM

Which one is better, Celeron g4900 or i36100? When it comes to the two processors Celeron G4900 and I36100, there is no doubt that the performance of I36100 is superior. Celeron processors are generally considered low-end processors and are primarily used in budget laptops. The I3 processor is mainly used for high-end processors, and its performance is very good. Whether you are playing games or watching videos, you will not experience any lagging when using the I3 processor. Therefore, if possible, try to buy Intel I-series processors, especially for desktop computers, so that you can enjoy the fun of the online world. How is the performance of the Celeron G4900T? From a performance perspective, the Pentium G4900T performs well in terms of frequency. Compared with the previous version, the CPU performance is

Power consumption testing and evaluation (rx588 power consumption testing and evaluation) Power consumption testing and evaluation (rx588 power consumption testing and evaluation) Dec 31, 2023 pm 09:38 PM

The power consumption of rx588 is about 250W. Watching movies on the rx580 consumes power, but not much. Many videos require hardware to decode. A graphics card will be used more or less. The platform power consumption of xfx Crimson and Dylan Demon + 7700K single card is about 380w, 7700k is counted as 100w, and other devices together are up to 30w, that is, the power consumption of rx588 is about 250w, and the power consumption of the dual-card machine is 630w to 650w , more than 700w is enough, but the load of the power supply is too high at this time, which affects the life. Calculated based on the load of 60% to 80%, the ideal power supply rated power is 850 to 1000w. It depends on what kind of CPU is used and whether it is overclocked. . Does h61 motherboard support 588 graphics card? H61 motherboard itself is

What is the difference between null and NULL in c language What is the difference between null and NULL in c language Sep 22, 2023 am 11:48 AM

The difference between null and NULL in C language is: null is a macro definition in C language, usually used to represent a null pointer, which can be used to initialize pointer variables, or to determine whether the pointer is null in a conditional statement; NULL is a macro definition in C language A predefined constant in , usually used to represent a null value, used to represent a null pointer, null pointer array or null structure pointer.

What do undefined and null mean? What do undefined and null mean? Nov 20, 2023 pm 02:39 PM

In JavaScript, both undefined and null represent the concept of "nothing": 1. undefined represents an uninitialized variable or a non-existent property. When a variable is declared but no value is assigned to it, the value of the variable is undefined , when accessing properties that do not exist in the object, the returned value is also undefined; 2. null represents an empty object reference. In some cases, the object reference can be set to null to release the memory it occupies.

How to fix missing MSVCP140D.dll issue on Windows How to fix missing MSVCP140D.dll issue on Windows Dec 22, 2023 am 10:51 AM

MSVCP140D.dll is a very important resource file. If an error occurs when running the program that MSVCP140D.dll is not specified to run on Windows, then you need to take it seriously, because if you do not handle it properly, the program will not be able to run. MSVCP140D.dll is not specified to run on Windows. Solution: Method 1: First download the msvcp140.dll file, decompress it and copy it to the corresponding system directory (the different directory addresses below correspond to different systems and cannot be mistaken) 1. For Windows95/98/Me system, copy it to the C:\Windows\System directory. 2.Window

What is the difference between null and undefined What is the difference between null and undefined Nov 08, 2023 pm 04:43 PM

The difference between null and undefined is: 1. Semantic meaning; 2. Usage scenarios; 3. Comparison with other values; 4. Relationship with global variables; 5. Relationship with function parameters; 6. Nullability check; 7. Performance considerations; 8. Performance in JSON serialization; 9. Relationship with types. Detailed introduction: 1. Semantic meaning, null usually means knowing that this variable will not have any valid object value, while undefined usually means that the variable has not been assigned a value, or the object does not have this attribute; 2. Usage scenarios, etc.

See all articles