• Explain exactly what > / dev / null 2 > & 1 is in shell


    preface I believe you can often find it in shell scripts>/dev/null 2>&1Such a statement. In the past, I didn’t understand the function of this command in depth, and copied it. Until last week, I accidentally wrote this command2>&1 >/dev/nullAfter a little problem, I began to understand the “mystery” behind this command. Introduction to shell redirection […]

  • Summary of null knowledge points in MySQL database


    In MySQL database, null is a frequent occurrence. What should be noted about null in MySQL? The following is a brief summary, which will be supplemented in the future. 1. is null First of all, judge whether the value of a column in the database is null. It can not be judged by equal, but […]

  • Solution to the problem that not in is null in MySQL


    Some time ago, when I was doing a small function in the company, I counted the number of data in a certain situation and then modified the problem. At that time, I thought it was very simple. I wrote the following SQL: SELECT COUNT(*) FROM t1 where tl.c1 not IN (SELECT t2.c1 FROM t2); The […]

  • New features of ECMAScript 2020


    Crazy technology houseOriginal text: https://www.strictmode.io/artNo reprint without permission ECMAScript 2020 is the 11th edition of our favorite programming language with some new features. Some are small features, but some will probably change the way we write JavaScript forever. Dynamic import() Es2015 introducedstatic importGrammar. Now you can export variables from one module and import them into […]

  • The correct use of null and space occupation in SQL Server


    We often encounter null in the use or maintenance of SQL server, so what is null? Here is a short description from MSDN (see “null values”): A value of NULL indicates that the value is unknown. A value of NULL is different from an empty or zero value. No two null values are equal. Comparisons […]

  • Solution to update field to null by JPA save() method


    This article mainly introduces the solution of JPA save() method to update the field to null. The example code is introduced in detail in this article, which has a certain reference learning value for everyone’s study or work. You can refer to the following for your friends Today, there is a problem in the development. […]

  • Summary of knowledge points related to null (ifnull, coalesce and nullif) in MySQL


    The examples in this paper describe the knowledge points of null (ifnull, coalesce and nullif) in MySQL. To share with you for your reference, as follows: In mysql, a null value represents an unknown value, which is different from 0 or the empty string ” and does not equal itself. If we compare a null […]

  • What is nulls in Java


    Preface For Java programmers, null is a headache. It is often disturbed by null pointer exception (NPE). Even the inventor of Java admitted that it was a huge mistake. Why should Java keep null? Nulls have been around for a while, and I think Java inventors know that nulls cause more trouble than the problems […]

  • Ajax method to determine whether the data returned by the back end is null


    1. Backend return: return null; 2. Ajax needs to be judged: if(null != jsonStr && “” != jsonStr){ Specific code: $.ajax( { type: “POST”, data: { name: name, code: code }, dataType: “json”, url: “info”, success: function(jsonStr) { if(null != jsonStr && “” != jsonStr){ $(“#curator_info”).html(“ok”); } else { $(“#curator_info”).html(“no”); } } } ); The […]

  • Some Suggestions on Setting Null in SQL Server


    When we design tables, we sometimes argue about whether fields allow Null values. Kalen Delaney, the cattle man in the database, gives some suggestions: 1. Never allow NULL values in user tables2. Include a NOT NULL restriction in the user table definition3. Do not rely on database attributes to control the behavior of NULL values […]