select * from a where aa=1 and bb=2;
select * from a where in (select id from a where aa=1 and bb=2);
the second feeling is superfluous. Performance mainly depends on whether there is an index on aa and bb
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select * from a where aa=1 and bb=2;but why do you write the second operation like that? There is no doubt about this. If you think about it, the second one will run on the basis of the first one. Who do you think is fast? There are certain prerequisites for reducing fields and improving performance efficiency based on primary key queries, not that performance can be optimized by using primary key queries as much as possible.
$selectTip = $pdo->query("SELECT * FROM `tips_rate` WHERE `tip_id` = ".$row[ id ]." " ); $selecttotal = mysqli_num_rows($selectTip); $rate = $pdo->query("SELEC...
for example, the isolation level of transaction An is repeatable read my understanding is that a read sql statement of transaction An is the same as the result repeated many times, regardless of whether or not other transactions have changed the data ...
I know that mysql master and slave have been configured, but I don t know what to do if one of them dies: 1. If there is only one master and one slave, do you want to upgrade the master manually in case the master dies? (it seems that you can t call...
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do a check-in function `create_time` decimal(20,3) NOT NULL, `create_date_ymd` varchar(15) NOT NULL COMMENT , `create_date_ym` varchar(15) NOT NULL, `create_date_y` int(4) NOT NULL, `create_date_m` int(2) NOT NULL, `create_date_d` int(2) ...
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1. Problem description assume that there are n game order tables in the database, and the table name is game_ [0-9a-z] + _ order. . It is known that there is an order number 18demo000001. How can we efficiently find out which game table this order is i...
I want to optimize a sentence about group by. I don t know where to start. I want to find a train of thought . SELECT adv_id, network_id, SUM(re_click) AS clk, SUM(deny_click) AS dny_clk, SUM(re_conver) AS re_ins, SUM(mat_conver) AS mat_ins, SUM(def_...
background: 1. I am a java programmer 2. Recently interviewed for a senior development position 3. The previous project mainly used less oracle,mysql, and the companies interviewed basically used mysql s 4. I am weak in the database 5. Recently, I...
like this SELECT * FROM material WHERE {key:value} limit 1 ...
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check-in table: fields include user id userid check-in time signtime timestamp type int check-in type type is divided into 0 and 1 for check-in and check-out, respectively. A user can sign twice a day, once check-in and one check-out. Check-in and ch...
Table structure ...
use mybatis to configure custom secondary cache. Using xml annotation can use < cache type= "com.test.dao.MybatisRedisCache " > to successfully use custom cache, but using @ CacheNamespace (implementation = MybatisRedisCache.class) on the dao i...
class essay extends Model { public function comment() { return $this->hasMany( App comment ) } } class comment extends Model { public function essay() { return $this->belongsTo( App essay ) } } ...
mybatis+mysql for java projects int batchAmount = 20000; for (int startIndex = 0; startIndex < size; startIndex += batchAmount) { int endIndex = startIndex + batchAmount; endIndex = (endIndex &...
Master and Slave built by mysql5.6, the newly created table is not backed up to the slave library master and slvae data can both correspond to can t the test library be automatically backed up to the slave library when establishing a master-slav...
I want to do a circular sequence to output pictures. Don t know how to write this code? Table structure for example, now the field ID of key=1 is 1, 2, 5, 7, 10, 11, 6 fields I want to output one at a time in order. output id 1 field for t...
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ask God to help me translate this native SQL statement into TP5 thank you! It is Db:: ( name )-> this kind of chained operation select a.id, a.titlerect a. Createwriting time as answer_count from fa_question a. Isometric solvepaper count (b.qid) as ...