In the process of enterprise development, I found that a colleague used the select * from where condition for update

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In the workplace Introvert

In the process of enterprise development, I found that a colleague used select * from where Conditions for update

ok ! Today I learned my first hand Send a post by the way . Hey

My article for reference

Novice tutorial -MySQL Business

Interviewer asked ：select…for update Will you lock the table or the row ？

Start bar ！ Hey

Look at a paragraph sql, Feel if you are confused There may also be big men

BEGIN;
SELECT * FROM a WHERE `key`=1 FOR UPDATE;
UPDATE a SET `name`='songxy9991' WHERE `key` =1;
COMMIT;

SELECT @@autocommit;
SET @@autocommit =0;

SELECT * FROM a WHERE `key`=1 FOR UPDATE;
COMMIT;
UPDATE a SET `name`='songxy12' WHERE `key` =1;

We explain one by one

• Here we are. We'll talk about This transaction Holy and great . Either successful or unsuccessful . Principle 、 Uniformity 、 Isolation, 、 persistence . Wait, Ba , And the isolation level of transactions , also spring Propagation characteristics of transactions in They are all regular interview sites

BEGIN

Is to start a transaction

COMMIT;

Commit transaction

SELECT @@autocommit;

Check whether the transaction is statically auto committed

autocommit State value description

• AUTOCOMMIT=0 Disable auto submit
• AUTOCOMMIT=1 Turn on auto submit
  modify state ：
• SET AUTOCOMMIT=0 Disable auto submit
• SET AUTOCOMMIT=1 Turn on auto submit
  MySQL By default, the database will be Automatically start the transaction

Here we are Don't let him commit transactions automatically Stop him first

SET @@autocommit =0;
SELECT @@autocommit;

The real Kung Fu begins Ha

• Create table statement

DROP TABLE IF EXISTS `a`;
CREATE TABLE `a`  (
  `key` int(0) NOT NULL AUTO_INCREMENT,
  `value` char(225) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NOT NULL,
  `name` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NULL DEFAULT NULL,
  PRIMARY KEY (`key`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 5 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_0900_ai_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of a
-- ----------------------------
INSERT INTO `a` VALUES (1, 'ssss', 'songxy12');
INSERT INTO `a` VALUES (2, 'yy', 'songxy');
INSERT INTO `a` VALUES (3, 'sxy', 'songxy');
INSERT INTO `a` VALUES (4, '1111', 'songxy');
INSERT INTO `a` VALUES (5, 'qqq', 'songxy');

SET FOREIGN_KEY_CHECKS = 1;

Let's take a look at the data in my previous table

First, check through the primary key

SELECT * FROM a WHERE `key`=1 FOR UPDATE;
 Execute this sql   I didn't  COMMIT; Ha 

In execution

UPDATE a SET `name`='songxy999' WHERE `key` =1;

Look up the table data

• No change

Execute the statement that commits the transaction

COMMIT;

• Is the lock OK or surface Let me try

• Wrong report ;error

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Look at the picture

This shows that the lock line

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